Material Basics – Stainless Steel
Introduction to Material Basics – Stainless Steel
Material Basics for commonly used material stainless steel, is covered in this blog. Which provides information about stainless steel material all commonly used grades, its properties, welding properties, including its subcategories like Austenitic, Martensitic, Ferritic, Duplex, Precipitation Hardening and super alloys.
Using Stainless steel when corrosion or oxidation is a problem is the most economical way to counter that. It was discovered around 50 years ago, that a minimum of 12% chromium would impart corrosion and oxidation resistance to steel.
Subsequently several important sub-categories of stainless steels have been developed. The sub-categories are Austenitic, Martensitic, Ferritic, Duplex, Precipitation Hardening and super alloys.
Austenitic grades are those alloys, which are commonly in use for stainless applications. They are not magnetic. The most common are iron-chromium-nickel steels and are widely known as the 300 series. The austenitic stainless steels, because of their high chromium and nickel content, are the most corrosion resistant of the stainless group providing unusually fine mechanical properties. They cannot be hardened by heat treatment, but can be hardened significantly by cold-working.
The straight grades of austenitic stainless steel contain a maximum of .08% carbon. There is a misconception that straight grades contain a minimum of .03% carbon, but the spec does not require this. As long as the material meets the physical requirements of straight grade, there is no minimum carbon requirement.
The “L” grades are used to provide extra corrosion resistance after welding. The letter “L” after a stainless steel type indicates low carbon (as in 304L). The carbon is kept to .03% or under to avoid carbide precipitation. Carbon in steel when heated to temperatures in what is called the critical range (800 degrees F to 1600 degrees F) precipitates out, combines with the chromium and gathers on the grain boundaries. This deprives the steel of the chromium in solution and promotes corrosion adjacent to the grain boundaries. By controlling the amount of carbon, this is minimized. For weldability, the “L” grades are used. You may ask why all stainless steels are not produced as “L” grades. There are a couple of reasons:
“L” grades are more expensive
Carbon, at high temperatures imparts great physical strength
Frequently the mills are buying their raw material in “L” grades, but specifying the physical properties of the straight grade to retain straight grade strength. A case of having your cake and heating it too. This results in the material being dual certified 304/304L; 316/316L, etc.
The “H” grades contain a minimum of .04% carbon and a maximum of .10% carbon and are designated by the letter “H” after the alloy. People ask for “H” grades primarily when the material will be used at extreme temperatures as the higher carbon helps the material retain strength at extreme temperatures.
You may hear the phrase “solution annealing”. This means only that the carbides which may have precipitated (or moved) to the grain boundaries are put back into solution (dispersed) into the matrix of the metal by the annealing process. “L” grades are used where annealing after welding is impractical, such as in the field where pipe and fittings are being welded.
The most common of austenitic grades, containing approximately 18% chromium and 8% nickel. It is used for chemical processing equipment, for food, dairy, and beverage industries, for heat exchangers, and for the milder chemicals.
Contains 16% to 18% chromium and 11% to 14% nickel. It also has molybdenum added to the nickel and chrome of the 304. The molybdenum is used to control pit type attack. Type 316 is used in chemical processing, the pulp and paper industry, for food and beverage processing and dispensing and in the more corrosive environments. The molybdenum must be a minimum of 2%.
Contains a higher percentage of molybdenum than 316 for highly corrosive environments. It must have a minimum of 3% “molybdenum”. It is often used in stacks, which contain scrubbers.
Restricts maximum carbon content to 0.030% max. and silicon to 0.75% max. for extra corrosion resistance.
Requires molybdenum content of 4.00% min.
Requires molybdenum content of 4.00% min. and nitrogen of .15% min.
Type 321 & Type 347
These types have been developed for corrosive resistance for repeated intermittent exposure to temperature above 800 degrees F. Type 321 is made by the addition of titanium and Type 347 is made by the addition of tantalum/columbium. These grades are primarily used in the aircraft industry.
Thermal Properties of Austenitics
Two thermal properties are critical in the design and fabrication of austenitic stainless steel products: high coefficient of expansion and low thermal conductivity.
While alloys of copper and aluminum have equal or higher (in the case of aluminum alloys) coefficients of expansion than austenitic stainless steels, it is the unique combination of high thermal expansion and low thermal conductivity that necessitates special precautions and procedures in the design and fabrication of stainless steel structures and vessels.
Distortion during welding
Failure to address thermal expansion and conductivity can result in severe distortion during welding, as differential expansion causes the heat generated by the welding process to remain localized, causing steep temperature gradients and high localized stresses or surface distortion. It follows that welding procedures should be adopted to minimize heat build-up in the weld zone. These procedures include using minimum amperage consistent with good weld quality, controlling interpass temperatures and, when feasible, using clamping jigs with copper or aluminum backing bars as heat sinks on the welds. Other precautions to minimize distortion during welding include efficient jigging or the use of a balanced sequence of closely spaced tack welds.
The Design Manual for Structural Stainless Steel indicates that austenitic stainless steels suffer from the same types of distortion during welding (angular, blowing, shrinkage, etc.) as carbon steel, but the higher coefficient of expansion (17 x 10-6/¡C versus 12 x 10-6/¡C for carbon steel) and the lower thermal conductivity (approximately 30% of carbon steel) increases distortion of austenitic stainless steel weldments.
The Design Manual also suggests that, while welding distortion can only be controlled and not eliminated, there are a number of actions to be considered by both the designer and the fabricator to minimize welding distortion, including those procedures detailed above. Other suggestions include designing with symmetrical joints, designing to accommodate wider dimensional tolerance, reducing cross-sectional area of welds in thick sections (e.g. replacing Single ‘V’ preparation by Double ‘V’ or Double ‘U’), ensuring that good fit-up and alignment are obtained prior to welding, and using balanced welding and appropriate sequences such as back stepping’ and block’ sequences.
Expansion problems after installation
Another problem arising from the high coefficient of expansion of austenitic stainless steels compared with carbon steels is that of differential expansion.
In the case of stainless steel bonded to plywood by adhesive, a maximum length of three meters is recommended to avoid failure of the adhesive bond during thermal cycling.
The replacement of carbon steel tubes with austenitic stainless steel tubes in some heat exchangers has resulted in failures due to differential expansion. In this instance it should be noted that expansion coefficients of ferritic and duplex grades of stainless steel are much closer to that of carbon steel and these grades are recommended for these applications.
In stainless steel piping systems, thermal expansion stresses can cause rupture of the support points, buckling of the pipe, or breakage of equipment connected to the piping if the changes in dimensions are not absorbed by expansion joints or flexibility of the piping installation. The Piping Manual for Stainless Steel Pipes for Buildings provides a guide to assessing thermal stresses and reactions at supports and anchor points, as well as a guide
to determining if the flexibility of piping can absorb its expansion. The latter involves an empirical formula which requires that the piping anchor points are at the pipe’s ends, the piping system has no branches, and there are no changes along the length of the pipe (e.g. diameter, thickness, material quality, temperature, etc.). If the flexibility cannot absorb the thermal expansion displacement, then expansion joints, flexible joints or ball joints should be used (after a computer stress analysis of the joint).
Evidently, thermal expansion and conductivity are critical determinants when designing and fabricating stainless steel products. Early consideration of these elements will ensure a better and longer-lasting product, both aesthetically and structurally
Martensitic grades were developed in order to provide a group of stainless alloys that would be corrosion resistant and hardenable by heat treating. The martensitic grades are straight chromium steels containing no nickel. They are magnetic and can be hardened by heat treating. The martensitic grades are mainly used where hardness, strength, and wear resistance are required.
Basic martensitic grade, containing the lowest alloy content of the three basic stainless steels (304, 430, and 410). Low cost, general purpose, heat treatable stainless steel. Used widely where corrosion is not severe (air, water, some chemicals, and food acids. Typical applications include highly stressed parts needing the combination of strength and corrosion resistance such as fasteners.
Contains lower carbon than Type 410, offers improved weldability but lower hardenability. Type 410S is a general purpose corrosion and heat resisting chromium steel recommended for corrosion resisting applications.
Has nickel added (2%) for improved corrosion resistance. Typical applications include springs and cuttlery.
Contains added phosphorus and Sulphur for improved machinability. Typical applications include screw machine parts.
Contains increased carbon to improve mechanical properties. Typical applications include surgical instruments.
Contains increased chromium for greater corrosion resistance and good mechanical properties. Typical applications include high strength parts such as valves and pumps.
Further increases chromium and carbon to improve toughness and corrosion resistance. Typical applications include instruments.
Ferritic grades have been developed to provide a group of stainless steel to resist corrosion and oxidation, while being highly resistant to stress corrosion cracking. These steels are magnetic but cannot be hardened or strengthened by heat treatment. They can be cold worked and softened by annealing. As a group, they are more corrosive resistant than the martensitic grades, but generally inferior to the austenitic grades. Like martensitic grades, these are straight chromium steels with no nickel. They are used for decorative trim, sinks, and automotive applications, particularly exhaust systems.
The basic Ferritic grade, with a little less corrosion resistance than Type 304. This type combines high resistance to such corrosives as nitric acid, Sulphur gases, and many organic and food acids.
Has lower chromium and added aluminum to prevent hardening when cooled from high temperatures. Typical applications include heat exchangers.
Contains the lowest chromium content of all stainless steels and is also the least expensive. Originally designed for muffler stock and also used for exterior parts in non-critical corrosive environments.
Has molybdenum added for improved corrosion resistance. Typical applications include automotive trim and fasteners.
Type 436 has columbium added for corrosion and heat resistance. Typical applications include deep-drawn parts.
Has increased chromium to improve scaling resistance. Typical applications include furnace and heater parts.
Contains even more chromium added to further improve corrosion and scaling resistance at high temperatures. Especially good for oxidation resistance in sulphuric atmospheres.
Duplex grades are the newest of the stainless steels. This material is a combination of austenitic and ferritic material. This material has higher strength and superior resistance to stress corrosion cracking. An example of this material is Type 2205. It is available on order from the mills.
Precipitation Hardening Grades
Precipitation hardening grades, as a class, offer the designer a unique combination of fabricability, strength, ease of heat treatment, and corrosion resistance not found in any other class of material. These grades include 17Cr-4Ni (17-4PH) and 15Cr-5Ni (15-5PH). The austenitic precipitation-hardenable alloys have, to a large extent, been replaced by the more sophisticated and higher strength super alloys. The martensitic precipitation-hardenable stainless steels are really the workhorse of the family. While designed primarily as a material to be used for bar, rods, wire, forgings, etc., martensitic precipitation-hardenable alloys are beginning to find more use in the flat rolled form. While the semi-austenitic precipitation-hardenable stainless steels were primarily designed as a sheet and strip product, they have found many applications in other product forms. Developed primarily as aerospace materials, many of these steels are gaining commercial acceptance as truly cost-effective materials in many applications.
Super Alloy Grades
Super alloys are used when 316 or 317 are inadequate to withstand attack. They contain very large amounts of nickel and/or chrome and molybdenum. They are usually much more expensive than the usual 300 series alloys and can be more difficult to find. These alloys include Alloy 20 and Hastelloy.