Introduction to Stainless Steel
What is Stainless Steel?
Stainless steel is a generic term for a group of corrosion resistant steels containing minimum of 10.5% of chromium. Varying additions of nickel, molybdenum, titanium, niobium and other elements may be present. The mechanical properties and behaviour in service of the various types depends upon their composition, and hence careful selection of the most appropriate type is vital to success in any application.
Types of Stainless Steel
There are five basic categories of stainless steel – Austenitic, Ferritic, Duplex, Martensitic and Precipitation Hardening. The austenitic grades are non-magnetic and in addition to chromium, typically at the 18% level, contain nickel (which improves fabrication and corrosion resistance) and are the most widely used group of stainless steels. The ferritic grades are magnetic, have a low carbon content and contain chromium as the main element, typically at the 13% and 17% levels. Duplex stainless steels have a mixed ferritic/austenitic structure. Chromium content varies from 18-28% and nickel from 4.5-8%. Duplex grades find application particularly where chlorides are present. The martensitic grades are magnetic containing typically 12% chromium and a moderate carbon content; are hardenable by quenching and tempering like plain carbon steels and find application mainly in cutlery manufacture, aerospace and general engineering. Precipitation hardening steels have chromium content at 17% typically with additions of nickel, copper and niobium. Precipitation hardening stainless steels have a minimum of 17% chromium and contain other elements that can provide strengthening through a precipitation hardening mechanism. They are capable of achieving very high strengths with proof stresses ranging in the 1000 - 1500 Mpa range depending on the type and the actual heat treatment.
Austenitic and ferritic grades account for approximately of stainless steel applications.
All stainless steels have a high resistance to corrosion. This remarkable resistance to attack is due to the naturally occurring chromium-rich oxide film present on the surface of the steel. Although extremely thin, this invisible, inert film is tightly adherent to the metal and extremely protective in a wide range of corrosion media. The film is rapidly self-repairing in the presence of oxygen and damage by abrasion, cutting or machining is instantaneously repaired.
In general the corrosion resistance of stainless steels improves as the chromium content increases. The addition of nickel raises corrosion resistance making these steels suitable for more aggressive conditions. The addition of molybdenum to the austenitic, terrific and duplex grades improves pitting corrosion resistance and produces the most resistant grades in these categories.
The lower alloyed ferritics are limited to mildly corrosive environment, often being used in less demanding applications.
Superior corrosion resistance and ease of cleaning results in widespread use of austenitic stainless steels in food and beverage equipment. Austenitic grades are also highly resistant to the wide range of rural and industrial atmospheres, resulting in extensive use in architectural and structural applications. Resistance to attack by acids, alkalis and other chemicals, has led to a wide use in the chemical and process plant industries.
Both ferritic and austenitic stainless steels are often found in household products.
Maintenance will usually involve simple cleaning methods to provide a clean, bright, attractive finish and lone service life.
Selection of the correct grade and finish is very important to obtain the corrosion resistance required. Aggressive corrosive media can necessitate the use of one of the more highly alloyed stainless steel grades.
The ferritic stainless steels can be welded in thin sections, but advice should be sought in welding practices. The austenitic and duplex stainless steels are readily welded by the full variety of welding techniques. Welding practices that incorporate high heat input and slow cooling rates, particularly on thick sections; can impair corrosion resistance adjacent to welds. To overcome this titanium stabilised grades and low carbon (L) grades have been developed. Brazing and soldering can be carried out successfully with light sections.
There is a wide range of stainless steels with other attractive properties to complement their corrosion resistance? the austenitic steels provide an example, Most austenitic grades have excellent formability and impact strength. The common grades can be used up to around 600°C or slightly higher if scaling resistance is the only consideration. More highly alloyed grades can raise this temperature appreciably with many furnaces using higher chromium and nickel grades up to 1100°C. They perform well at cryogenic temperatures, retaining excellent ductility and impact properties. Cold working and forming processes considerably increase the strength and hardness of austenitic grades, a property exploited in springs and structural components. The addition of nitrogen to austenitic and duplex grades increases strength and hardness which are retained after welding or heat treatment. Selection of the correct grade is thus important to give the best combination of properties for the application.
Stainless steel grades have designations derived from various sources. “18/8” or “l8/10” for instance is a non-technical description often seen describing grade 304 or its equivalents.
a. AISI three digit designations such as 304 or 316 are now the most widely used system, being adopted, sometimes with minor variations by Standards bodies. Japan, Britain and Australia. They are also used in ASTM specifications which tend to be used, or at le understood in almost every country. The system proven durable, but inadequate to cope with new alloys. Extra letters designate alloy variants, such as 304L, 316LN or 310S.
b. UNS numbers address the inadequacies of the AI system. One letter ("S" for wrought stainless steels, for cast) plus five digits based on the AISI numb Grade 304 is UNS S30400, 304L is S30403 and 304H is S30409. Grade 904L is officially classified as a nickel based alloy hence designated UNS N08904.
c. European designations originated as German Werkstoff Numbers (eg 1.4301 for Grade 304) or DIN grade designations leg X5CrNi 189 for 304). Similar descriptive designations were used in other European countries. These are now incorporated in “Euronorm” specifications which replace national specification throughout the European Union.
d. Proprietary designations have often been given alloys by their original manufacturer. Standard designations are later added when alloys are widely accepted, but the proprietary designations often persist. Examples are "253 MA" which is an Avesta Sheffi trademark (UNS equivalent is S30815) and "SAF2304” (which is a Sandvik trademark (UNS equivalent S32304).
e. Cast alloys are often loosely referred to by equivalent wrought designations, but a cast "304" has different composition and properties. The section on Castings gives further details.
Alloying Elements in Stainless Steel
Stainless steel is a steel to which a minimum 10.5% chromium and other alloying elements are added. The addition of different elements influences the mechanical, physical and corrosion resistant properties of stainless steel. Listed below is a summary of the effects of the most important alloying elements.
In most stainless steels carbon (C) is kept to low levels, typically 0.05% C, or 0.03% C in the low carbon "L" grades and other stainless steels used for fabricated components in thicker sections. However, in martensitic stainless steels C is the alloying element added, in amounts varying from 0.15% - 1.2% C, to render these steels heat treatable by quenching and tempering to develop high strength and hardness levels.
A minimum of 10.5% Chromium (Cr) is needed in stainless steel, at which level a continuous, stable and inert (passive) chromium oxide film forms on the surface, producing resistance to corrosion, both wet (aqueous) and dry (gaseous). Higher levels of Cr (up to about 26% Cr) further increase corrosion resistance.
Nickel (Ni), if added to stainless in sufficient quantity, develops a fully austenitic crystal structure, hence austenitic stainless steels, as typified by grade 304 (18% Cr 8% Ni). Lower levels of Ni, insufficient to develop a fully austenitic crystal structure, can result in a duplex (mixed) crystal structure of ferrite and austenite (found in duplex stainless steels).
Molybdenum (Mo) enhances the properties of the passive surface film, and renders stainless steels which contain it more corrosion resistant, particularly to pitting in chloride environments. The higher the Mo content the more aggressive the corrosive conditions that can be handled (eg grade 316 with 2% Mo and grade S31254 with 6% Mo).
Titanium (Ti) is added to produce stabilised grades such as 321. Titanium is a strong carbide former and preferentially forms Ti carbides obstructing the formation of Cr carbides. This activity prevents intergranular corrosion occurring in the region adjacent to welds.
Manganese (Mn) like Ni promotes the formation of an austenitic crystal structure. In the 200 series austenitic stainless steels Mn is used to partially replace Ni. Min is also increased to slightly higher levels than normal in the free machining grades of stainless steel to which sulphur (S) or selenium (Se) have been added.
Sulphur (S) is normally kept to very low levels, typically 0.03% maximum, but in practice much lower. If increased to around 0.2% S (as in grade 303) the machinability, of the steel is improved, but the fabricational, mechanical and corrosion resistant properties are impaired.
Niobium & Tantalum
Niobium (Nb) and Tantalum (Ta) are stabilising elements with effects similar to Ti. Grades incorporating Nb and Ta are seldom used, the Ti stabilised grades being preferred. However, Nb is used for the manufacture of welding consumables (grade 347 in both electrodes and i1ler wire), as Ti tends to be lost in transfer across the arc.
Nitrogen (N) promotes an austenitic crystal structure, and is used to complement Ni in the "N" grades of austenitic stainless steels. The yield strength of such grades at subzero temperatures is vastly improved. N is also used in duplex stainless steels (eg S31803) to increase the austenitic fraction of the crystal structure. improving weldability.
Silicon (Si) is added to improve the scaling resistance of austenitc heat resistant stainless steels (such as 314 and S30815). In castings higher Si content increases the fluidity of the molten metal improving the "castability" and high Si GMAW (MIG) welding wire is usual as the Si promotes washing and wetting behaviour.
Useful Facts About Stainless Steel
Conditions Which Favour Use of Stainless Steel
Key Considerations in Working Stainless Steels
- Corrosive Environments
- Very low (cryogenic) temperatures — prevents brittleness
- High temperatures — prevents scale maintains strength
- High strength vs mass
- Hygienic conditions required — easy cleanability
- Aesthetic appearance — no rust, thus no paint necessary
- No contamination — prevents catalytic reactions
- Discharge slideability from hoppers
- Wet abrasion resistance
- Non-magnetic properties of austenitic grades
- Know the material: this improves decision making, avoids problems and saves costs.
- Know the grade of material: Connect material selection is vital risk taking is costly.
- Know the design: Good design ensures savings for fabricator and user.
- Know surface finishes: Good finishes perform wall look good and promote sales.
- Know your supplier
- Apply good housekeeping: Good housekeeping saves rectification costs.
- Apply accurate identification: Lost identity can prove costly.
- Apply production planning: Planning saves costs and promotes quality
All stainless steels have a much lower conductivity than that of carbon (mild) steel (plain chromium grades approximately ¼). This must be borne in mind for any operation which involves high temperature, e.g. effects during welding (control of heat input), longer times required for heating to attain a uniform temperature for hot working.
Plain chromium grades have an expansion coefficient similar to carbon (mild) steels, but that of the austenitic grades is about one and a half times higher. The combination of high expansion and low thermal conductivity means that precautions must be taken to avoid adverse effects, e.g. during welding use low heat input dissipate heat by use of copper backing bars and use adequate jigging. This factor must also be considered in components which use a mixture of materials, e.g. a heat exchanger with a mild steel shell and austenitic grade tubes.
Stainless steels rely on a very thin surface passive film for their corrosion resistance. It is vital to maintain and preserve the integrity of the passive film.
- Avoid mechanical damage and contamination.
- Repair any affected areas (e.g. high temperature scale adjacent to a weld, mechanically damaged or ground areas, contaminated areas), by passivation only or by both pickling and passivation
- Ensure a constant and sufficient availability of oxygen at the surface of the stainless steel
Stainless steels have a tendency to gall, pick-up or seize. To avoid this take precautions such as:
Stainless Steel — High Quality, but NOT indestructible
- For surfaces experiencing relative motion - minimise the load, ensure no heat build up, keep free of grit or contaminants, use lubricants or surface coatings.
- On threaded components the threads must have a high degree of surface finish, components should have an intermediate to free fit. avoid over?torque and contamination of the threads.
DO store away from carbon steels.
DO use storage racks and handling equipment protected by: plastic, dry timber, felt, etc., which will not mark the surface of the steel or permit contact with carbon steel.
DO handle with care and avoid damage to the product’s shape.
DO use clean gloves
WHEN supplied on product, keep the protective plastic or strippable coating intact as long as possible?it helps prevent damage in the period between delivery and final installation.
DON'T allow contact with carbon or other steels, especially use of tools or equipment used on carbon steel.
DON'T walk on the steel.
DON'T scratch the steel.
DON'T allow contact with foreign substances, e.g. fingerprints, oil, grease.
DON'T expose protective coatings, when applied to the steel, to direct sunlight unless special UV resistant marks are on the coating.