posted 12-25-2000 07:29 PM              

Stainless steels are defined as those iron base alloys which contain at least 11 percent chromium. The chromium provides the oxidation and corrosion resistance by forming a tenacious film of chromium oxide. Other elements are added to improve corrosion resistance and strength. There are five classifications of stainless steels: austenitic, ferritic, martensitic, duplex and precipitation hardening. There are a number of alloys within each classification and most of these alloys are weldable by the GTAW process when correct procedures are followed. Approximate parameters for welding the stainless steels are given in Table 5. Filler wire composition should be as recommended by the manufacturer for each grade of steel.

Austenitic Stainless Steel

The basic austenitic stainless steel contains 18% Cr and 8% Ni commonly known as 18-8 or AISI 304. Some variations have up to 26% Cr and 22% Ni with small amounts of elements such as Mo, Ti, Cb and Ta. As the name implies, these steels remain austenitic at room temperature and to very low temperatures and are characterized by good ductility, toughness and weldability. However, several factors must be considered to make good welds. First, the austenitic stainless steels have a high coefficient of thermal expansion which is over 50% greater than that of the carbon steel and low alloy steels. This can cause high shrinkage stresses which can lead to warpage and hot cracking. These conditions can be minimized by use of low total heat input and use of joint designs which minimize the amount of filler material used. The low thermal conductivity is a help because weld heat is dissipated less rapidly than for carbon steel. Strong fixtures can reduce distortion but may develop higher residual stresses.

The austenitic stainless steels are susceptible to chromium carbide precipitation in the heat affected zone which occurs between 420 degrees and 870 degrees Celsius. This action which is also known as sensitization removes chromium from solution in the metal at the grain boundaries and reduces the corrosion resistance. These areas can be attacked by some corrosive media and result in severe intergranular corrosion. The remedy for this problem is to weld with low heat input to shorten the time in the sensitization temperature range, use low carbon stainless steel and filler wire or use stabilized stainless steels that contain Ti, Ta and Cb which react with carbon leaving the Cr in solution.

Another problem is hot-short cracking as the weld is cooling. Although base metals are normally 100% austenite, it has been found that a weld deposit containing 3 to 10% ferrite will be more resistant to hot cracking than 100% austenite filler wire. Thus, filler wire which will produce a weld deposit with ferrite in this range will avoid hot cracking. Ferrite level can be measured with an instrument called a Magnegage or calculated from the composition using the Schaeffler, DeLong or WRC 1988 diagrams. No more ferrite should be used than needed to avoid cracking because ferrite can form Sigma phase (an iron chromium intermetallic compound) on long time service exposure at 540-870 degrees Celsius. Sigma phase reduces ductility, toughness and corrosion resistance.

Ferritic Stainless Steels

The ferritic stainless steels are primarily Fe-Cr alloys which contain enough Cr and other ferrite stabilizing elements to prevent the formation of austenite on heat treatment or welding. Since there is no transformation of these steels on heating, they are not heat treatable and cannot be hardened. The earlier Fe-Cr ferritic stainless steels contained enough carbon to make them susceptible to intergranular carbide precipitation in the weld heat affected zone unless a post weld solution heat treatment is used. Later ferritic stainless steels contained additional ferrite formers, carbide formers and very low carbon, which reduced the susceptibility to carbide precipitation.

The main problem encountered in welding the ferritic stainless steels is grain coarsening in the heat affected zone and loss of toughness. This occurs because there is no transformation of austenite on heating and no subsequent transformation back to ferrite to provide grain refinement. Normally no preheat is required because there is no transformation and preheat would increase the degree of grain coarsening. However, the earlier Fe-Cr steels with the higher carbon levels can form some austenite on heating which can form martensite on cooling and lead to cracking under high restraint. A preheat of 150 degrees Celsius or higher can minimize stresses and cracking. Filler wires for welding these steels normally should be of matching or nearly matching compositions. For dissimilar metal joints, either austenitic stainless or nickel base alloy filler wires should be used.

The GTAW process should be used with DC electrode negative and argon, helium or a mixture of these for shielding. AC welding is not recommended.

Martensitic Stainless Steels

The martensitic stainless steels also are Fe-Cr alloys but contain less Cr and more carbon than the ferritic steels to allow them to transform to austenite on heating and subsequently transform to martensite on cooling. These steels are hardenable and the hardness depends upon the carbon content. The high chromium content makes them air hardenable so that a fully martensitic structure will be obtained at even slow cooling rates. When the carbon is high enough, this will produce a heat affected zone of high hardness and low toughness. These steels can be welded in any heat treated condition, (annealed(a), hardened(b), stress relieved(c), or tempered) because the heat treated condition has little affect on the behaviour of the heat affected zone hardening and thus the weldability. Weldability depends upon carbon content and decreases as carbon increases. The best way to avoid cracking is to use the proper preheat and interpass temperature and post weld heat treatment. Preheat and interpass temperature in the range of 200-320 degrees Celsius is recommended. After welding, the weldment should be cooled to about 200 degrees Celsius to allow transformation to martensite then be heated to the tempering or annealing temperature without cooling below 200 degrees Celsius to prevent formation of cracks. preheat temperature will depend mainly on the carbon content and also on joint thickness, filler material and degree of restraint.

Types 410, 410 Ni Mo and 420 martensitic stainless steels are available commercially as standard filler metals. Other proprietary grades are also available commercially. Type 410 is recommended for welding types 403, 410, 414 and 420 grades. Type 410 Ni Mo is used to weld type CA-6NM castings to similar grades. Type 420 can be welded with ER420 filler metal when matching carbon content is needed. Since the martensitic weld metals exhibit poor toughness in the as-welded condition, they must be post weld heat treated to provide adequate weld toughness.

Austenitic stainless steel filler metals such as types 308, 309 and 310 can be used to weld the martensitic stainless steels to themselves or to other grades of stainless steel to provide a greater as-welded toughness than martensitic filler metal. It must be realized, however, that austenitic stainless fillers may give tensile and yield strength less than matching filler metal and post weld heat treatment.

Annealing - Heating to and holding at a suitable temperature and then cooling at a suitable rate for the purpose of reducing hardness to facilitate machining or cold work.

Hardening - Increasing hardness by suitable heat treatment, which usually consists of heating and cooling to one or more temperatures.

Stress Relieving - Heating to a suitable temperature, holding long enough to reduce residual stress and then cooling slow enough to avoid development of new residual stresses. Stress relieving is not intended to reduce hardness significantly.

Duplex Stainless Steels

The duplex stainless steels typically consist of a microstructure of 50 percent ferrite and 50 percent austenite but may range from 20-80 percent ferrite. Compositions are modified from the typical austenitic compositions by increasing chromium to 22-26 percent, decreasing nickel to 4-8 percent, adding molybdenum to 2-5 percent and copper up to 2 percent. The desirable 50 percent ferrite - 50 percent austenite microstructure is obtained after a heat treatment, hot working or a combination of these treatments. Normal heat treatment is in the range of 1030-1150 degrees Celsius which causes part of the original delta ferrite to transform to the more stable austenite.

The duplex stainless steels provide excellent resistance to pitting, crevice corrosion and stress corrosion cracking. Ductility and toughness approach those of austenitic stainless steels and yield strength is almost twice that of wrought austenitic stainless steel.

A weld made with matching composition filler material or without filler material will exist primarily as ferrite unless followed by heat treatment or hot working. The high ferrite welds tend to be very brittle and generally will crack on cooling before any post weld heat treatment can be applied. Since it would not be practical to post weld heat treat or hot work most large weldments, it is necessary to use a filler metal which will provide an as-deposited microstructure of about 50 percent austenite and 50 percent ferrite. Recommended filler metals for the duplex stainless steels are of matching compositions except that nickel is increased to 8-10 percent. For GTAW of Ferralium 255, ER2553 is recommended and for Sandvik 2205, ER2209 is recommended.

Welding of duplex stainless steels of 6mm and greater thickness normally would be done by an arc welding process other than GTAW to utilize the greater deposition rates of the other processes. Root pass welds and welds in thin sheet could be made by the GTAW process using solid filler wire. Heat input must be carefully controlled to minimize dilution to obtain the desired level of austenite in the deposit.

Precipitation Hardening Stainless Steels

The hardness and strength of the precipitation hardening stainless steels can be increased by a solution and precipitation (ageing) heat treatment. There are three grades of precipitation hardening stainless steels within this group of which the hardness of each can be increased by the ageing mechanism. Martensitic grades can be hardened by the martensite reaction then hardened further by ageing. Semiaustenitic grades must be given a conditioning heat treatment which will allow martensite to form on cooling. Ageing will provide additional hardening. the third grade is austenitic which remains austenitic to well below room temperature. It can be hardened only by the ageing method.

Weldability of the martensitic grades is generally excellent. They are not crack-sensitive or susceptible to hot cracking because carbon content is low. Hot cracking may occur if they are welded to carbon or low alloy steels where dilution is excessive. Preheating is not necessary to avoid cracks. Heavy sections and parts of highly restrained weldments may be heat treated to a tough overaged condition prior to welding.

Weldability of the semiaustenitic grades is good and no preheating or post weld heat treatment is required. These grades can be welded in any condition without reheating or control of interpass temperature since the weld metal and heat affected zone remain austenitic in the as-welded condition. When filler metal of similar composition is used and recommended procedures are followed, weld strengths between 90 and 100 percent of the base metal strength can be expected.

The austenitic grades are the most difficult to weld of the three grades of precipitation hardening stainless steels. The base material should be in the solution treated condition, and multipass welding with small stringer beads using the GTAW process is recommended.

The GTAW process or other inert gas shielded processes are recommended for welding all grades of precipitation hardening stainless steels to avoid loss of easily oxidized elements such as Al, Ti and Cb.