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Soil corrosivity is often classified by its resistivity. 0 – 1500 ohm.cm very corrosive 1500 – 2500 ohm.cm corrosive 2500 – 5000 ohm.cm mildly corrosive 5000 – 10000 ohm.cm slightly corrosive > 10000 ohm.cm non-corrosive Experience suggests that there is little likelihood of corrosion of 304 and 316 stainless steel with soil resistivities of 2000 ohm.cm and above if the pH is greater than 4.5 and there is clean drainage and backfill. As with other environments, chloride levels in relation to pitting and crevice corrosion may also influence the performance of stainless steel in soils and where higher chloride levels are anticipated inland or for non-tidal coastal areas or just where a greater degree of confidence is required, type 316 would be preferred. For coastal areas and other situations where a greater degree of resistance may be required, 2205 duplex stainless steel or super duplex stainless steel can be considered. If thought necessary, external protection such as appropriate protective casings or tapes (taking care to ensure an effective overlap to avoid crevice corrosion) may be used. Stainless steel can also be cathodically protected. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The continued increase in semiconductor device sophistication, and resultant decrease in line widths, require cleaner components throughout process gas delivery systems. Properly designed high purity gas filter assemblies are transparent to homogeneous constituents in the gas stream, i.e., to remove particulate contamination with no other effect on the inlet gas purity. In order to accomplish this goal, the assemblies used to house the filter devices are most often constructed of smooth, clean and electropolished 316 stainless steel. “Smooth and clean” is a relative term, requiring more explicit definition. Specification requirements are established whenever performance limits for filter assemblies are clearly understood and measurable. For example, moisture contribution and surface roughness are reasonably well defined. However, these criteria are based on measurements limited by the sensitivity of the best available instrumentation. In order to meet these criteria, the “best” available raw materials from which to fabricate components are often specified. The logic in simply picking the best material follows only if the material ultimately meets the intended specification requirements and if the filter assembly manufacturer understand which material properties are governing. wilsonpipeline carefully evaluates all raw materials used to fabricate semiconductor gas filter assemblies. There are many important characteristics of 316L stainless steel that must be considered in designing these high purity assemblies. Type 316L stainless steel is a well characterized and understood alloy. Its good corrosion resistance is a result of both careful formulation of the major alloying elements (Fe, Ni, Cr, Mn, and Mo), as well as the control of critical trace constituents (C, S, and Si). Similar to other finished components, filter assemblies utilize stainless steel which has been further worked, i.e., rolled, drawn, machined, or welded.  In these cases, it is not sufficient to merely specify material chemistry. Each intended use of the stainless steel may entail slight modification of the basic specification, but the most commonly specified requirements resulting in a fine, electropolished surface finish are: balanced 316L chemistry. homogeneous austenitic microstructure. very few inclusions. fine grain structure. Materials Comparison: Stainless Steel Bar HEAT ID# SPEC.–» N/A SA479 35439 Typical N/A VAR Spec.Typ.1 ELEMENTAL(%): Fe BAL. BAL. BAL. Cr 16-18 17.1 16-18 Ni 10-14 12.0 10-14 Mo 2-3 2.1 2-3 Mn 2* 1.7 <2 Si 1.0* 0.45 ≈0.5 C 0.03* 0.020 <0.03 S 0.03* 0.0020 ≤0.005 * = Maximum Additional Criteria GRAIN SIZE — 5 Not Specified2 INCLUSION TYPE (ASTM E-45) — NUMBER NUMBER Sulfides — 0.5 2.03 Alumina — 0 03 Silicates — 0 2.53 Oxides — 1.5 1.53 1 316L VAR does not specify (or result in) chemical composition which is of higher performance than SA479 with the noted exception of reduced sulfur. 2 Typical measured values were within a window of 5-10.  3 Not specified. Typical indicated. In order to remove microscopic voids in the melt, as well as to further purify the metal by reducing certain contaminant impurities, many high purity stainless steels undergo a secondary melting process (e.g. vacuum arc remelt, known as VAR). This process reduces sulfides, silicates, phosphates, aluminas, and other impurities. Of these impurities, sulfur is the most detrimental to the achievement of fine surface finishe (assuming the other impurities are within normally specified limits).  These trace impurities tend to disturb (“break”) the stable austenitic structure and form inclusions which are less noble than the parent metal. When the inclusions are infrequent and very small in size, this may not be a problem. However, when very fine surfaces (<10 microinch Ra) are desired, all inclusions become apparent after final mechanical polishing or electropolishing. It is for this reason that 100X microscopic inspection after fine mechanical polishing is commonly used to quantify the inclusion content. Electropolishing often accentuates the defects in the surface by removing the less noble inclusions first, via preferential dissolution. Grain structure must also be considered when choosing high purity stainless steel. Fine grained materials are preferable since they tend to be more homogeneous. As grains grow (e.g during slow cooling), there is more time for the austenite breakers to precipitate at the grain boundaries. This has the effect of transforming a clean dislocation of the structure (“healthy” grain boundary) into a site for inclusions. In addition, fine grained materials conduct electric current more uniformly. Uniform conductance is an important characteristic during the electropolishing process.

Stainless steel is selected for architectural application, for their corrosion resistance and potential aesthetic appearance. For internal applications other factors also important, including: – Risk of the fingermarking from contact with ‘human traffic’ (on wall cladding and lift panels) risk of scratching or staining from contact with glasses or drinks (bar tops) can routine cleaning be expected to maintain the surface appearance but not cause any unacceptable changes in appearance Selection of stainless steel grade With the exception of harsh internal environments often encountered in leisure and hydrotherapy pool buildings, grade selection is not a critical factor. For most building interiors intended for human occupation, either the ferritic 430 (1.4016) or austenitic 304 stainless steel can be considered. Depending on surface finish, there can be slight natural colour differences between them, which could influence the choice. The ferritic steel can have a blue tinge, whilst by comparison, the nickel containing 304 stainless steel has a slight yellow tinge. Surface finishes There is a wide range of possible finishes. Normally ‘ex-mill’ finishes, with the exception of the bright annealed, 2R finish, may not be considered as aesthetically suitable as textured, polished and coloured finishes. British Standard EN 10088-2:2005 defines these basic ‘special’ finishes in table 6. Specifying finishes to EN 10088-2. The finishes include: Patterned, Corrugated, Mechanically Polished or Brushed, Coloured Mechanically polished or brushed finishes This includes a wide range of possible finishes, including designations ‘G’, ‘J’, ‘K’ and ‘P’ of EN 10088-2. Brushed finishes may be better choice than ‘ground’ or ‘polished’ for preventing fingermarking. As the austenitic 304 stainless steel is not particularly hard, highly polished surfaces can be susceptible to damage. Where surface scratching is very likely on bar or counter tops, then a uniform (non-directional) polish may be better than directional finish. Unless contaminated with iron from normal carbon steel, however, scratches on stainless steel do not detract from theircorrosion resistance in such application areas. This is due to the ‘self-healing’ nature of the passive surface layer on the steel. Coloured finishes Colours including blue, black, bronze, charcoal, gold, green and red are available on 316 and 304 stainless steel types. These colours can be applied to mill, patterned, polished, bead blasted or etched surface finishes and offers a wide range of colour and texture. The colour does not contain dyes or pigments, but relies on light interference path difference effects in a chemically thickened surface passive layer. Generally colouring is done on tube sheet material before fabrication and so the scope for complex shapes or forms may be limited. Surface scratches are difficult to repair and so this range of finishes may not be suitable for high traffic areas and is better suited to cladding or roofing applications. Patterned finishes Cold rolled embossed, three-dimensional patterns to either one or both sides. These ‘2M’ finishes help mask scratches and marks and so are useful in high traffic areas, especially lift doors, fascias and lining panels. Tube Sheet with patterned rolled finishes, depending on the pattern depth, can exhibit improvements in stiffness for panel applications.

Stainless steel grades, such as the 304 (1.4301) or 316 (1.4401) types are generally suitable for storing and handling cold or unheated drinking town’s waters. Localised corrosion by crevice or pitting mechanisms is not usually a hazard in properly designed, fabricated and finished tanks handling clean waters of drinking quality. Hot water tanks however may be at risk from stress corrosion cracking (SCC). Stress corrosion cracking (SCC) risk factors. The factors that influence SCC attack are: temperature, chlorides, tensile strength, oxygen level For these types of applications chlorides and oxygen levels are fixed by the water chemistry, but chlorides can concentrate in splash zones or at the water /air line by evaporation. This can also be a hazard if external insulation to tanks becomes wet.Temperature should be fixed by the tank system controls, but hot spots can be a problem, especially if chlorides concentrate, as described. A design and fabrication method with as few ‘engineering crevices’ as possible is advisable as this reduces the risk of stress concentrations and also guards against crevice corrosion attack. Fully filled welded joints are preferable to seams with laps or mechanically fastened joints. Controlling residual stresses The main design and fabrication factor that can be ‘controllable’ is stress. Residual tensile stresses can be a cause of SCC failure. Relief of these stresses can be advisable where high level of residual stresses are possible or the application is critical. A range of treatments for the 304 and 316 type austenitic can be considered: – ‘sub-critical’ stress relief treatment, e.g. 450C and slow cool ‘full-anneal’ e.g. 1050-1100C and ‘quick’ cool (air) Stress relieving austenitic stainless steel The restraining effect around welds on austenitic stainless steels such as 304 or 316 types can also be a source of tensile stresses. If it is impractical to post fabricate heat treat, then control of welding parameters may help. These would include: – Careful pre-weld tacking Minimising heat input during welding by controlling welding speed Alternatives to the austenitic stainless steel to reduce the risk of scc failure Alternatives to the austenitic, which have nickel levels making them particularly susceptible to SCC are either: Ferritic stainless steel Duplex stainless steel These types have lower nickel levels and so are more resistant to SCC as a result of the ferrite phase present. (Higher nickel level alloys are also more resistant to SCC, but are not considered economically justifiable for these applications.) The ferritic grade 444 (1.4521) or duplex grades 1.4362/S32304 or 1.4462/2205 have been considered as alternative SCC resistant grades for hot water applications. Availability and cost in the UK could be factors prevailing against these types. Forming and welding differences from the austenitic stainless steel must also be borne in mind when making the steel selection. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Stainless steel is widely used in foods and beverage manufacturing and processing industries, bulk storage and transportation, preparation and presentation applications. Depending on the grade of stainless steel selected, they are suitable for most classes of food and beverage products. This artical includes an extenive section on stainless steel. Stainless steel used in food processing Most containers, pipework and food contact equipment in stainless steel is manufactured from either 304 or 316 type austenitic stainless steel. The 17% chromium ferritic stainless steel (type 430) is also used widely for such applications as splashbacks, housings and equipment enclosures, where corrosion resistance requirements are not so demanding. In addition to these non-hardenable austenitic and ferritic types higher strength ‘duplex stainless steel’ types, such as grades 1.4362 and1.4462 are useful for ‘warm’ conditions where stress corrosion cracking (SCC) can be a corrosion risk, such as in brewery sparge tanks. Hardenable martensitic type stainless steel is widely used for cutting & grinding applications, especially as knives. Is 316 type the only stainless steel that is classed as the ‘food’ grade The 316 grades (1.4401 / 1.4404) are often referred to as the food grades. S32101 (1.4162) is very similiar with 316. There is no known official classification for this and so, depending on the application, the equally common 1.4301 and 1.4016 grades may be suitable for food processing and handling, bearing in mind that in general terms the corrosion resistance ranking of grades can be taken as:  1.4401/1.4404 (316 types) > 1.4301 (304 types) > 1.4016 (430 types) Corrosion hazards to stainless steels in food processing If the grade of stainless steel is correctly specified for the application, corrosion should not be encountered. Surface finish and condition is very important to the successful application of stainless steels. Smooth surfaces not only promote good cleansibility but also reduce the risk of corrosion. The types of corrosion to which stainless steels can be susceptible are summarised below. This can be useful in identifying problems due to wrong grade selection or inappropriate use of equipment. Pitting Corrosion and Crevice Corrosion Both crevice corrosion and pitting corrosion occur most readily in aqueous chloride-containing solutions. Although attack can occur in neutral conditions, acidic conditions and increases in temperature promote pitting and crevice corrosion. Pitting corrosion is characterised by local deep pits on free surfaces. Crevice corrosion is occurs in narrow, solution-containing crevices or sharp re-entrant features in a structure. Examples of potential sites for crevice corrosion are under washers, flanges and soil deposits or growths on the stainless steel surface. Stress Corrosion Cracking ‘SCC’ is a localised form of corrosion characterised by the appearance of cracks in materials subject to both stress and a corrosive environment. It usually occurs in the presence of chlorides at temperature generally above 50�C. Intergranular Corrosion ‘IGC’ or ‘ICC’ (known in the past as ‘weld decay’) is the result of localised attack, generally in a narrow band around heat affected zones of welds. This is more likely to occur in the ‘standard’ carbon austenitics. The risk of IC attack is virtually eliminanted if the low carbon (0.030% maximum, eg 1.4307) or the ‘stabilised’ (eg 1.4541) types are selected. Cleaning of stainless steel equipment Effective cleaning is essential in maintaining the integrity of the process and in prevention of corrosion. The choice of cleaning method and the frequency of its application depends on the nature of the process, the food being processed, the deposits formed, hygiene requirements etc. The cleaning methods listed are suitable for stainless steel equipment. Water and SteamMechanical ScrubbingScouring Powder and DetergentsAlkaline SolutionsOrganic SolventsNitric Acid Disinfection of stainless steel equipment Chemical disinfectants are often more corrosive than cleaning agents and care must be exercised in their use. Hypochlorites Hypochlorites, chloramine and other disinfectants can liberate free chlorine, which can cause pitting. Sodium hypochlorite or potassium hypochlorites are often used in commercial sterilising agents. If these substances are used with stainless steel, the duration of the treatment should be kept to minimum and followed by thorough rinsing with water. At higher temperatures, chloride-containing sterilising agents should not be used with stainless steel. Milton solutions (hypochlorite & chloride) can be very aggressive to stainless steels. Tetravalent ammonium salts Tetravalent ammonium salts are much less corrosive than hypochlorites, even when halogens are present in their formulation. Iodine Compounds Iodine compounds may be used for the disinfection of stainless steel. Nitric acid Even at low concentrations, nitric acid has a strong bactericidal action and can be a low cost disinfectant for stainless steel equipment, especially in dairies and pasteurising equipment. Maintenance of food process equipment Stainless steel equipment often contains gaskets or other components that can absorb or retain fluids. These liquids may be become concentrated by evaporation and corrosion may ensue. Equipment should be disassembled occasionally for thorough cleaning. If the disassembled equipment exhibits corrosion (crevice corrosion usually), then the corroded surfaces should be cleaned. Typical applications of the various stainless steel types TypesTypical Applications420 (martensitic)Cooks and professional knives, spatulas etc430 (ferritic)Table surfaces, equipment cladding, panel (ie components requiring little formability or weldability). Used for moderately corrosive environments (e.g. vegetables, fruits, drinks, dry foods, etc).304(austenitic)Vats, bowls, pipework, machinery parts (i.e. components requiring some formability or weldability). Corrosion resistance superior to 430.316(austenitic)Components used with more corrosive foods (e.g. meat/blood, foods with moderate salt contents), which are frequently cleaned, with no stationary solids and not under excessive stress.904L 1.4539(austenitic)Used with corrosive foods (e.g. hot brine with solids that act as crevice forms, stagnant and slow moving salty foods).1.4462(duplex)Used with corrosive foods (e.g. hot brine with solids, stagnant and slow moving salty foods). Higher strength than austenitics. Good resistance to stress corrosion cracking in salt solutions at elevated temperatures.6%Mo. types (austenitic)Used with corrosive foods (e.g. hot brine with solids, which act as crevice formers, stagnant and slow moving salty foods). Good resistance to stress corrosion cracking in salt solutions at elevated temperatures. Used in steam heating and hot work circuits, hot water boilers, etc Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The specification of stainless steel bar (to BS970 EN 10088-3 ) and stainless steel tube plate (to BS1449) before 1983 covered two type 316 grades: a ‘low’ carbon with 0.03% max (316S12) and a ‘standard’ carbon with 0.07% max 316S16. Both had a molybdenum content in the range of 2.25-3.0 %. The 1983 and subsequent versions of the standard introduced new molybdenum content ranges of 2.0-2.5% and 2.5-3.0% for both the low and ‘standard’ carbon grades, thereby increasing the number of these grades from two to four: 316S11 low carbon, lower molybdenum range 316S13 low carbon, higher molybdenum range 316S31 standard carbon, lower molybdenum range 316S33 standard carbon, higher molybdenum range The chromium range was maintained across all of the grades at 16.5-18.5%, but the nickel range was adjusted to ‘balance’ the structure of these austenitic stainless steel. Nickel is an ‘austenite stabiliser’ and so is used to balance the combined effects of carbon (also an austenite stabiliser) and molybdenum which has the opposite effect as a ‘ferrite stabiliser’. The only equivalent for the old 316S12 is the 316S13, so that the minimum molybdenum of 2.25 is met. Similarly, the only equivalent for the old 316S16 is the 316S33. For atmospheric exposure service conditions, except in extreme marine environments, the substitution of either 316S11 for 316S12 or 316S31 for 316S16 should give satisfactory corrosion resistance, depending on design and finishing techniques. Pre 1983 specifications to BS970-1 and stainless steel tube plate (to BS1449) 316S12316S16% min% max.% min% maxC 0.03 0.07Si0.201.000.201.00Mn0.502.000.502.00Ni11.014.010.013.0Cr16.518.516.518.5Mo2.253.02.253.0S 0.030 0.030P 0.045 0.045 Post 1983 specifications to BS970-1 and stainless steel tube plate (to BS1449)316S11316S13316S31316S33% min% max% min% max% min% max% min% maxC 0.030 0.030 0.07 0.07Si 1.0 1.0 1.0 1.0Mn 2.0 2.0 2.0 2.0Ni11.014.011.514.510.513.511.014.0Cr16.518.516.518.516.518.516.518.5Mo2.002.502.503.002.002.502.503.00S 0.030 0.030 0.030 0.030P 0.045 0.045 0.045 0.045 Specification to EN 10088-2 and stainless steel bar (to EN 10088-3 ) In 1995 BS EN 10088-2 replaced BS1449 and BS EN 10088-3 replaced BS 970-1. The four 316 sub-grades have numerical designations as follows:Former BS designation316S11316S13316S31316S33Current BS EN numbers1.44041.44321.44011.4436 Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The raw material cost for a particular component may be 20 times the cost if made from one material compared to another on a weight to weight basis. However the lifetime costs may be very similar if all of the other factors are also taken into consideration. The material cost of a mass produced investment casting item may be 80% of the final cost. The material cost of a single complicated machined item may be less than 10% of the final cost. It is not possible to provide cost comparisons between different metals to any level of accuracy. Each metal is varying in price on a day to day basis and different alloys of the same metal can have significantly different costs. A grade 7 titanium alloy costs twice as much as pure titanium (grade 1,2 or 3). Comparing costs should only be based on final installed costs. eg. for a domestic, industrial piping system a screwed steel system would cost about 40% more than a copper piping system. Example : The price of a titanium / titanium alloy products results from a number of factors: Alloying grade .some grades e.g with Pd alloying component, can significantly increase the price of the alloy. The purity of the grade… the more pure the higher the cost The test and inspection requirements; The procured quantities. The more ordered the lower the specific cost The geometry ..rolling or forging affects prices per volume or weight Demand ..e.g High defence demand for aerospace industry can result in higher metal prices Local economy.. Metal availability In year 2000 the price of titanium was about 13 000 to 43 000/tonne.. In 2002 the price of raw titanium was about to 8960/tonne. In 2005 to-date the price of titanium has varied between 6000 and 9000 /tonne Ref 2010 ..I have enclosed a chart from the metalprices webpage to illustrate to range of titanium ingot pricing over a 12 month period Table showing relative metal costs The table below can only really be used to give broad relative initial material costs.  The figures are based on a reference source originating about 2002 MaterialDensityCost/tonneRelativeCost /m3Relativekg/m3/tonne/tonne/m3/m3Carbon Steel7820550143011,0Alloy Steels78208301,516490,61,5Cast Iron72258301,515996,751,4Stainless Steel778044508,134 6218,0Aluminium alloys270022204,059941,4Copper Alloys8900555010,149 39511,5Zinc alloys710022204,015 7623,7Magnesium alloys180040007,372001,8Titanium alloys450017 00030,976 50017,4Nickel alloys890018 00032,7160 20036,8 Current Metal Prices.. November 2010 I have tried to obtain some current (Nov 2010) metal prices from various internet sources and I list them below..These sometimes differ considerably from the table above MaterialCost/tonneRelative Cost (weight)Relative Cost (volume)/tonneSteel (Billet) LME-Nov-201032111Steel (Hot Rolled Plate)-MEPS-July-20105051,61,6304 Steel (Hot Rolled Plate)-MEPS-July-20102 5367,97,9316 Steel (Hot Rolled Plate)-MEPS-July-20103 5351111Tin- LME-Nov-201015 4584845Aluminium Alloy – LME-Nov-20101 4074,.41,5Aluminium – LME-Nov-20101 4254,41,5Copper – LME -Nov-20105 27916,418,7Zinc – LME -Nov-20101 4124,44,0Nickel – LME -Nov-201014 3984451Lead – LME-Nov-20101 4144,46,4Titanium (ingot 6AL-4V) steelonthenet (11$/lb)15 7004928 Prices in dollars for 25mm round bar x 300mm long (0,00015m3 ) …November 2010 (Onlinemetals (USA)MetalPrice $ 1000/tonne 1000/m3 Relative cost (weight)               Relative cost (volume)                 Steel HR (A56)63,224,711Steel CD (12L40)7,43,930,51.231.23Alloy Steel (4130)9,995,341,21.661.66St.Stl (304L)15,348,163,22.62.56St.Stl (316L)20,6110,984,93.43.44Aluminium (2011-T3)8,5813,135,44.21.43Copper (C110)37,9117,5156,25.66.32Brass (C360)25,311,7104,33.74.22Bronze42,0919,5173,56.27.0Titanium (6AL-4VG5)107984413117.8 Comparison of raw bulk material costs above with actual metal stock prices Looking at the cost per tonne of HR steel in the form of a 1″ dia round bar 1 foot long at 3200 against the LME price for the raw material in bulk for at 321/tonne illustrates the massive diiference in the price of raw materials to the actual price of materials as supplied for initial machining processes in small quantities.

Stainless steel is probably the least understood. Selecting the right stainless steel for your application can be perplexing. Here we tell you differences between various stainless steel alloy. Hope help you to select the most appropriate material for specific application. The focus of this issue is on the stainless steel and related alloy that combine a high degree of hardness,corrosion resistance, low cost and availability. It is intended for the designer need to specify material on the drawing. Stainless Steel 52100 The alloy is homogeneous so that all of the chromium is available for corrosion resistance. The alloy melts clean and heat treats beautifully so high cycle fatigue strength is good and the alloy supports high rolling contact stress. Good quality stock is easily purchased. QC requires checking decarb in as-received stock and after heat treating checking decarb and grain size. Stainless Steel 440C It is hard. The alloy has big, blocky, primary carbides. The matrix chromium is only a little higher than H13’s so its corrosion resistance is only a little better than H13. The primary carbides make for noisy rolling contact, markedly lower rolling contact fatigue strength, fairly good compressive strength, and poor tensile properties. There are very consistent supplies of good quality. QC requires monitoring the heat treater’s results for excessive austenite grain boundary precipitation, prior austenite grain size, primary carbide particle size and retained austenite. Stainless Steel H13 Not as clean as 52100 so its rolling contact fatigue strength isn’t quite as high but its higher chromium gives pretty goodcorrosion resistance. Hard to buy good quality and hard to heat treat well. QC requires monitoring incoming steel for segregation and monitoring the heat treater’s results for austenite grain size and precipitation in the austenite grain boundaries. Stainless Steel 17-4PH This compromise has better corrosion resistance, good toughness when properly heat treated, and lower hardness. Coarse grain can be a problem. Commercial stocks seem to be good quality. QC requires monitoring the heat treater’s results for austenite grain size, precipitation in the austenite grain boundaries and through-thickness hardness. Hardness after the solutionizing heat treatment needs to be checked, which is the usual as-purchased condition for small quantities. Segregation can be a problem. Stainless Steel 420 The alloy has good corrosion resistance and pretty good hardness. However, it heat treats with a coarse grain, which makes it somewhat unpredictable and reduces the fatigue strength and rolling contact stress. It is moderately easy to purchase good quality. QC requires monitoring segregation and grain size after heat treating. Hard worked Stainless Steel 304  High hardness and strength with pretty good corrosion resistance are available in heavily cold worked type 304. It has much less toughness than annealed 304 but it is right up there with the other hard stainless steel tube. Wire, small stainless stel bars and small strip dimensions are available. Quality control includes surface finish, which can be scaly on a microscopic scale. Other hard stainless steel include: 416 for improved pitting resistance 440A and 440B for better homogeneity than 440C but still suffer from primary carbides 17-7PH for more of the same as compared to 17-4PH The growing family of fully densified powder metals gives great opportunities for combining all the properties except affordability. In a critical application, where the metal cost can be absorbed, be sure to look over what’s available. We see these fully densified powder metals in a frustratingly limited range of small stainless stel bar stock sizes. An example of compacted powder metal stainless stel bar stock is Crucible Materials Corporation’s CPM T440V which has 17% chromium along with other good things. The powder process keeps the carbides fine as long as the heat treater doesn’t mess it up during final heat treating after machining. The heat treater’s results must be monitored for carbide and grain size or the product isn’t any better than the regular ingot and strand cast mill product.

Stainless steel is selected for architectural applications, as with most other applications, for their corrosion resistance. This is usually the prime consideration. Environmental factors such as temperature and humidity need to be taken into account, but the location of the proposed site is the initial consideration. The Nickel Institute’s ‘Stainless Steel in Architecture, Building and Construction Guidelines for Corrosion Prevention’ publication categorizes sites as either: Rural,Urban,Industrial,Marine Definitions of sites Rural sites are defined as unpolluted, inland sites away from industrial atmospheres or discharges. Urban sites are defined as residential, commercial or light industrial areas with non-aggressive airborne pollution, typically from road traffic (exhaust fume and winter road salt spray may be issues). Industrial sites are typified by airborne pollution such as sulphur dioxide or gases released from chemical process plants, which can form potentially dangerous acid condensates. Marine sites are defined as areas where windborne sea spray or mist may be present. These contain chlorides which can also concentrate in condensates or as surface moisture evaporates. Local micro-climates and changes to the enviroment The environment cannot usually be defined precisely in these terms and it also important to bear in mind that environmental changes may occur during the design life of a proposed building ie. is the environment getting more polluted or cleaner, for any given location? Additionally ‘micro-climates’ can influence the general categorisations and may be worth investigating for any proposed site before a final stainless steel grade selection is made. Microclimates can exist in coastal locations or near chemical plant chimneys, where unexpected acid condensates can form. Sub-pisions of the ‘site-types’ should also be considered. Low temperatures and low humidity reduce the risks of corrosion and can mean that a steel grade perhaps not thought suitable for a particular site may be worth considering. Selection of stainless steel grades Selection guidelines are summarised in the table. Only the ‘common’ 304 (1.4301) and (1.4401) 316 stainless steel types are considered as candidates for most UK sites. .RuralUrbanIndustrialMarine.LMHLMHLMHLMH31633332222122130422222111X21X The ‘local’ conditions are defined as: ConditionsLLeast corrosive conditions e.g. low humidity and low temperaturesMTypical atmospheric conditions for the site typeHHarsh atmospheres, typified by persistent high humidity, high temperature or high levels of pollution The performance ratings are defined as: Performance Rating3Probably over-specified, for corrosion resistance requirements and cost2Probably the best choice for corrosion resistance and cost1Worthy of consideration if precautions are taken (i.e. good standard of surface finish and regular cleaning specified)XLikely to suffer severe corrosion This shows that the 304 (1.4301) type can be considered for most sites, except either heavily polluted industrial sites or most marine sites. In these cases the (1.4401) 316 stainless steel types should be the preferred choice. Life expectancy for stainless steels in external environments Natural rain washing of the items should be considered an advantage, as the corrosion risk from pollutants or condensates is reduced. Similarly, exposed sections are less likely to hold condensation due to the improved natural ‘ventilation’ available to the steel surfaces. Additional factors for consideration Other important factors in stainless steel selection are: – Surface finishDesignFabrication methodsAccessibility for cleaning and maintenanceMechanical properties and physical properties of stainless steels. Surface finish As a general rule, the smoother the finish, the better the corrosion resistance. Selection of polished surface finishes often requires a considerable amount of work before a final agreement is reached. This may involve having swatch samples prepared and agreed by the specifying parties. Polished finish K of BS EN 10088-2 is noted in the standard, Table 6 as being intended for external architectural applications, but is only one of many options. Highly reflective finishes may not be advisable especially for roofs, as this could be a hazard to air traffic on buildings near airports or on flight paths. Alternative dull finishes have been developed for such applications. Reflective finishes can be used to advantage however to reflect light into dark, enclosed courtyard areas of buildings. Patterned finishes are better for hiding scratches and fingermarks in ‘high traffic’ areas. Coloured finishes are also available for special aesthetic affects. Design Crevices must be avoided, as these can be sites for localised corrosion. Fabrication methods and corrosion hazards Fabrication methods that avoid crevices should be considered. Mechanical fixings can introduce crevices both at the fastener and at the lapped metal joint. Aluminium fasteners (e.g. rivets) should be avoided for securing stainless steel panels, as galvanic corrosion to the aluminium can be a problem in harsh environments. Avoid moisture traps at any mechanically fastened joints. Contact with lead or copper should not result in galvanic corrosion, but staining to stainless steel parts from the patina may be visible if rain water drains over the stainless steel. Sealants can be considered to avoid such problems. Adhesive bonding, if mechanically strong enough, usually eliminates such problems. Welds should be full seam welds, rather than intermittent fillet welds. Compatible welding consumables should be specified with full penetration weld designs, where possible. Iron contamination during storage and erection MUST be avoided. This is a common cause of unnecessary rust staining and attendant remedial post hand-over costs. Mortar cleaning (hydrochloric) acids must not be allowed to come into contact with stainless steels. Accessibility for cleaning and maintenance Periodic cleaning is advisable on stainless steel, as with most building exterior materials. The frequency will depend on local conditions and the ‘visibility’ of the steelwork. Where cleaning and maintenance is difficult or costly, e.g. on the outside of high rise buildings, then a more resistant grade selection than suggested by the tables may be appropriate. Mechanical and physical properties of stainless steels The mechanical properties of the commonly used 304 and 316 stainless steel types do not usually present a cause for concern. The thermal expansion rates of these grades however is about a third as much again as most steels. i.e. around 16 x 10-6 /C compared to around 12.2 x 10-6/C for carbon steels. Expansion joint allowances must account for this to avoid thermal buckling problems and any sealants used must be compatible.

Salt spray testing is an accepted method for assessing the suitability of stainless steel parts and fabrications that are likely to encounter chloride environments in service. For any particular part, tested under laboratory conditions, a difference in performance between say, 430 (1.4016), 304(1.4301) and 316 (1.4401) types stainless steel would be expected, but the test outcome is sensitive to the shape of the parts (designed-in crevices), surface finish and the test conditions. Using specific laboratory salt spray test data to assess if a particular stainless steel grade is suitable for a specific or ‘generic’ environment is therefore not appropriate. Salt spray test methods Salt spray testing is covered by the standards such as: ASTM B 117 – Practice for Operating Salt Spray (Fog) Testing Apparatus Tests durations may be up to 260 hours. BS7479:1991 – Method for Salt Spray Corrosion Tests in Artificial Atmospheres.  This supercedes BS5466:1977 and is equivalent to ISO 9227. The test duration’s range from 2 to 96 hours. The pass / fail criteria is that thers should not be any ‘visible’ staining on parts tested. The test acceptance criteria can therefore be subjective and need to be clearly defined for any set situation. This standard covers methods in three types of atmospheres: NeutralAcetic acidCopper accelerated acetic acid (CASS) Comparison of 316, 430 and 304 type stainless steel in salt spray testing Results from one testing laboratory, based on tests done to the ASTM B117 method suggest a 316 type stainless steel part could be expected to pass a 96 hour test using a 3% salt spray. Indications are that longer test times would not be expected to give satisfactory results. In contrast, 304 type stainless steel parts would not be expected to give satisfactory results in a 3% salt spray, but if the salt solution concentration is reduced to 0.3 %, then it is possible that a 304 type stainless steel parts may be satisfactory for test times up to 120 hours. This could also apply to 430 type ferritic or 431 type martensitic stainless steel parts. Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The high thermal conductivity of copper can not compensate poor thermal conductivity of  organic solid black particles resulting in decrease of heat transfer. Table 1 demonstrates the effect of fouling on thermal resistance of copper and stainless steel tubes with wall thickness of 0.049”. These calculations show that thermal resistance of stainless steel tubes in the fuel oil preheater will be less then thermal resistance of copper tubes, accumulated coked hard crust deposit of hydrocarbons. The thermal resistance of the tubes is only a part of total resistance of heat transfer, and the effect of fouling and tubing material can be accurately calculated in the process of the thermal design of a heat exchanger. CopperStainless Steelk- Btu/(hr ft deg. F)225.008.4t/k – (hr sq. ft deg. F)/Btu0.00001810.0004861Fouling 0.0050.005   (per TEMA)Total0.00501810.0054861Per Cent100%109.3% Where:  t/k-Thermal Resistance of the tubes, t-wall thickness, k- coefficient of thermal conductivity. Regretfully, TEMA Recommended Good Practice values of Fouling Resistances for Industrial Liquids do not take in consideration effect of tube material. If the actual Fouling Resistance of # 6 Fuel Oil in the heater made from stainless steel tubing equals to 0.0025, then:Fouling0.0050.0025Total0.00501810.0029861Per Cent100%   40% The coking of hydrocarbons accelerated at elevated temperature of heating tubes. The evaporation of light hydrocarbon residue fractions definitely accelerates coking. As a good practice, The Alstrom Corporation recommends to maintain the temperature of heating media about 120 deg. F above the outgoing temperature of the #6 heating oil or less. For instance, #6 Fuel Oil Outgoing Temperature, deg.FRecommended Temperature of Heating Media, deg.FMaximal Steam Pressure, psig120240101502703018030050220340100250370150 The excessive steam pressure can be used to size temperature regulator. These recommendations are valid for asphalt and other highly viscous liquids. For instance, heating syrups may result in carmelization of the fluid. Similar, but in less degree, phenomena occurs in shell & tube heat exchangers for water heating. Indeed, copper tubes are oxidized, resulting in fast accumulation of solid deposits. Copper has poor mechanical properties, particularly on elevated temperatures common in heat transfer. Tube-to-tube sheet joint of copper tubes with other materials commonly made by tube expansion. ASME Boiler & Pressure Vessel Code estimates efficiency if expanded joint 60-65%. Stainless steel tubes can be not only expanded but also seal welded to the tubesheet, resulting in 100% joint efficiency.  In this case, according to the Standard for Power Plant Heat Exchangers of Heat Exchange Institute, the metal temperature of welded joint can reach the maximum value permitted by ASME Code. After introducing 316 stainless steel tubes as a standard material of fabrication shell & tube heat exchangers and using expansion-welding technology The Alstrom Corporation was never reported about tube damage or leak in the tube-to-tubesheet joint.

The causes of disappointment can arise at any point in the long supply chain that often applies to a stainless steel project. This helps to explain why problems occur. Getting the appropriate knowledge to all parts of the supply chain is difficult and it only takes ignorance in one small part to create a problem later on. Importance of surface finish in determining corrosion resistance 1. Lack of knowledge in this area is a major cause of problems. Most specifiers and designers understand the importance of selecting a grade of stainless steel, for example 1.4301 (304) or 1.4401 (316). But surface finish is at least as important. The subject is fully explored in The Importance of Surface Finish in the Design of Stainless Steel. Briefly, a bright polishedsurface gives maximum corrosion resistance. A directional polish equivalent to the EN 10088-2 2K (Ra = 0.5 micron max), usually produced using silicon carbide (SiC) abrasives, will give adequate corrosion resistance in many severe environments notably heavy urban and coastal ones. A common surface finish achieved with 240 grit alumina abrasives has been implicated in the corrosion of stainless steel in urban and coastal environments. In some cases, surface roughness Ra values have been measured at well above 1 micron which is known to be inadequate in these environments. The lack of any specified surface finish on architectural drawings can be the source of the final problem. If, at any stage of the supply chain, there is any doubt about the appropriate surface finish, specialist advice should be sought. 2. Importance of post-fabrication treatments Apart from some specialised processes, welds in stainless steel always result in some degree of heat tint. Heat tint is essentially an oxidised surface which has a reduced corrosion resistance compared to the parent material. Therefore, the normal practice is to carry out some form of post weld treatment to improve the corrosion resistance. Details of these procedures can be found at: Post Weld Cleaning and Finishing of Stainless Steels Welding and Post Fabrication Cleaning for Construction and Architectural Applications Good fabrication practice always includes post weld treatment. Failure to do so can give rise to unnecessary cost of rectification later on. 3. Importance of segregating carbon and stainless steel Sometimes “rusting” of stainless steel turns out to be nothing of the kind. It is the rusting of carbon steel which has contaminated the surface of the stainless steel at some point in the production process. Possible sources of contamination from carbon steel include: Tools Lifting Gear, Ropes, Chains Grinding dust Cutting sparks Wire brushes Wherever possible, stainless steel and carbon steel should be fabricated in separate areas of the  workshop or better still in separate workshops. Where not possible it is important to clean down machines used for carbon steel before using them for stainless steel. Stainless steel surfaces should be protected with plastic coatings for as long as possible. 4. Importance of site management It is quite possible for everything to be done well in fabrication, only for the whole project to be spoiled by inappropriate practices on site. The issues outlined in 3. apply just as much to the site installation as anywhere else in the process. In addition, it must be remembered that what is appropriate for one building material is totally  unacceptable for another. For stainless steel it has to remembered that masonry and brick cleaners may contain hydrochloric acid sometimes called muriatic acid. If these fluids are to be used at all near stainless steel, care should be taken to protect the stainless steel surfaces. If splashes occur, they should be immediately washed off with water. Failure to do so will result in serious attack of the stainless steel resulting in expensive rectification costs 5. Importance of choosing correct grade for the application This aspect almost goes without saying. It is only this far down in the list because it usually is  considered. But if the “wrong” grade has been chosen the consequences can be severe. Some guidelines on material selection are given here. 6. Cleaning and Maintenance Some people think that stainless steel’s corrosion resistant surface somehow repels dirt and other contaminants. Like any surface stainless steel requires some maintenance. Guidance on this aspect can be found at Cleaning Methods for Stainless Steel Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)