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1 ) Austenitic Stainless Steel
It is one of the most widely used stainless steel types and preferred in food industry, pharmacy, fermentation, chemistry as well as petro-chemistry.
AISI 304 and AISI 316 quality steel types are included in this group. ‘304’ contains 18% chrome and 10% nickel. These stainless steel types have a perfect corrosion resistance. ‘316’ is an austenitic stainless steel type which contains 17% chrome, 12% nickel and 2.2% molybdenum. They can be used in much more serious corrosive areas such as chloride environments where ‘304’ remains incapable. .
Another important feature of austenitic stainless steel is that it has no magnetic characteristic in opposition to general steel types.
These stainless steel types contain 12%~ 25 Cr and 8%~ 25 Ni. As nickel has a strong austenite structure, the austenite arising on these steel types during the hardening process remains untransformed even at temperatures lower than the room temperature. It cannot be hardened by quenching as Austenitic does not turn into Ferrite. The most known steel type in this group is the one which is called 18/8 steel and contains 18% Cr as well as %8 Ni. This stainless steel type is anti-magnetic and some Molybdenum is added to it for increasing its corrosion resistance. We can list here the most important features of austenitic stainless steel in terms of its welding ability.
Its heat transmission coefficient is 1/3 of low alloy and plain carbon steel.
Its thermal expansion coefficient is 50% higher than plain carbon and low alloy steel.
Unalloyed carbon steel has a low electric conductivity resistance, but this value is 5-7 times higher in these stainless steel types.
Chrome-nickel steel shrinks more than plain alloy steel during welding because of its abovementioned characteristics. The internal stresses observed on welding seams during the cooling process of these seams as a result of high shrinking cause cracking risks. Hot cracks are highly possible to occur on double-sided inside corner seams of these stainless steel types. A partially martensite structure is obtained on these steel types through extreme cold transformations especially hammering. A carbon precipitation tendency is observed especially when 18/8 type austenitic stainless steel is heated up to 450 ~ 850oC and held at this temperature.
‘C’ content of austenitic stainless steel should be maximum 0.6% and preferably 0.03%. Stress relief annealing is applied on chrome-nickel stainless steel at times after the welding. Annealing temperature is selected as 800 ~ 920oC. Table 7 and 8 show the chemical composition, mechanical features and temperature degrees of austenitic stainless steel types.
A series of metallurgical factors play an important role on the welding ability of austenitic chrome-nickel stainless steel in addition to its physical characteristics affecting this ability. These are; the constitution of ferrite phase, sensitivity to inter-granular corrosion, sensitivity to stress corrosion and the constitution of sigma phase.
During the production of austenitic chrome-nickel stainless steel, austenite and d-ferrite granules start to constitute when hardening begins as of the liquid phase. This is different from the ferrite which arises after the transformation of austenite. Hardening naturally occurs on the d-ferrite granules scattered among the austenite granules which form the steel structure. This phase is rich in the elements balancing chrome and ferrite while it is poor in the elements balancing nickel and austenite. Steel producers don’t want this phase to take place: it makes the hot working of steel difficult and encourages the constitution of cracks on the material. This phase’s permanent existence on granule boundaries reduces the corrosion resistance. In addition, facing with the d-ferrite phase for a long period at high temperatures may cause the constitution of a hard and brittle sigma phase which has a reducing effect on the resistance and formability of the material.
A secondary problem occurring during the welding of austenitic chrome-nickel stainless steel is the chrome carbide precipitation tendency arising when some chrome-nickel steel types, especially 18/8, remain at the temperature range of 450-850oC for a long period of time. These steel types are cooled in a fast way after reaching 1100oC where chrome carbide dissolves in austenite. Thus, the precipitation risk of these elements is eliminated and precipitation cannot take place during the use of these steel types as the diffusion velocity of carbon is so slow at room temperature. When the temperature raises up to 450oC, diffusion velocity of carbon increases to a level which can take carbon out of its grain boundaries. The carbon element accumulating on grain boundaries constitutes chrome carbide (Fe, Cr23 C6) by combining with chrome in this area because of its high affinity against chrome. As 90% of this chrome carbide’s weight consists of chrome, even the little amount of carbon on grain boundaries reduces the chrome amount around the austenite granules (Figure 1). As a result of this, corrosion occurs on the grain boundaries with reducing chrome amounts as the material is in a corrosive environment. The intergranular corrosion taking place in this way makes the material unusable in a very short time. This incident intensifies as steel’s carbon content increases.
As the area melting during the welding of austenitic chrome-nickel stainless steel hardens and cools rapidly in very short time and as the carbon content of the alloys used as electrode is low, there is no carbide precipitation risk for the welding metal, namely the welding seam. However, the area influenced by the heat remains annealed at the temperature range of 500-900oC during the welding period and the carbide precipitation incident to cause intergranular corrosion on austenitic grain boundaries in case of a high carbon content takes place as this area is also the base metal. The intensity of this carbide precipitation is bond to temperature and time for certain carbon content. A temperature and changing incubation period is observed before the beginning of precipitation. There is a temperature level where precipitation begins within the shortest time according to the temperature and steel’s carbon content and this is called ‘critical temperature’.
Austenitic chrome-nickel stainless steel is sensitive to hot cracking. This situation reveals itself especially during the arc welding which is performed with covered electrode. We can list the precautions to be taken and the points to be taken into consideration in this situation as follows:
The smallest electrode diameter must be selected,
The lowest current intensity must be used,
Zigzag moves mustn’t be applied on the electrode and passes should be applied gently,
In multi-pass welding, the part must be cooled down to room temperature after each pass is applied and second pass must be applied after that. It must be cooled as fast as possible within the bounds of possibility.
The crater at the end of welding must definitely be filled in, any crack detected during the welding must be removed by grinding and welding must be applied after that.
2 ) Martensitic Stainless Steel
Although these stainless steel types have much better resistance ability in proportion to others, they have the lowest stainlessness ability. They are generally used in the areas where high resistance and hardenability features are required together in addition to stainlessness.
Martensitic stainless steel types can contain about 0.3% of carbon while austenitic stainless steel and ferritic stainless steel types contain 0.02-0.04% of carbon. These stainless steel types can be hardened by quenching thanks to their high carbon contents. AISI 420 quality steel belongs to this group. It contains about 0.2% of carbon and about 13% of chrome.
Stainless steel types included in this group generally contain less than 16% of Cr; the carbon amount included in their composition varies between 0.5% and 1.2%. Cr amount of the steel types with high C amounts can rise up to 18%. Martensite composition takes place very slowly since they have a low cooling velocity (in calm weather).
They have a perfect corrosion resistance when they are in martensitic condition. They don’t lose their stainlessness characteristics up to 815 0C. If they are exposed to heat for a long time, an incipient corrosion takes place. So, they are not used at temperatures higher than 700 0C in the industry.
Stress relief anneal is applied to these steel types at 650 0C while softening anneal is applied at 825 0C. Low carbon martensitic stainless steel types are welded after certain precautions are taken. High carbon type mustn’t be welded. Martensite amount of low carbon martensitic stainless steel types is relatively less and hard, accordingly their cracking tendency is lower.
These steel types are usually subjected to preliminary annealing before the welding. This preliminary annealing does not reduce the hardness in the area affected by the heat as in high C equivalent steel types. Only the cracking possibility reduces as the thermal stress reduces too. The most appropriate annealing temperature range for the preliminary annealing of these steel types is 200 ~ 400oC. To eliminate the cracking possibility just after the welding, steel parts must be subjected to stress relief anneal after the welding process when possible. They must be annealed at 820 ~ 870oC for a period of 4 hours, their temperature must be reduced to 590oC in the furnace in a very slow way and then they must be cooled in the warm weather.
For the welding of austenitic stainless steel types, austenitic welding metal (austenitic electrodes) is used when the resistance of welding seam is not so important and the steel part is not in a sulfurous environment. The low yielding point of austenitic welding metal eliminates the cracking risk caused by the shrinking tensile constituting after welding. Martensite stainless steel types with high C contents (0.5% - 1.2%) cannot be welded in a safe way despite all these precautions.
Martensite stainless steel types are used in valves, fittings, gears, pins, load transfer shafts and chains while low carbon types are used in turbine blades and wheels as well as steam turbines. They have no magnetizing characteristics.
3 ) Ferritic Stainless Steel
These steel types have a lower stainlessness characteristic in proportion to austenitic stainless steel types. So, they are rather used in fields such as automotive industry where serious levels of stainlessness characteristic are not required. Having more brittle resistance features in proportion to austenitic stainless steel types, these steel types have some magnetic characteristics which cause a disadvantage for stainless steel. Some types contain 11.5% - 16.5% of chrome, but they contain less than 0.5% of nickel.
These stainless steel types contain 16~30 % of Cr and 0.25~0.5% of C. The most important features of these steel types are that; they can be hardened by quenching as they don’t constitute a phase transformation in solid condition and they have a high corrosion and oxidation resistance at high temperatures.
These steel types can only be hardened through cold forming. So, they are not used very much as the hardness caused by cold forming, although it is in small amounts, make it difficult to form the steel. These steel types are subjected to soft annealing at 750 ~ 800oC in order to eliminate the cold forming hardness.
They can be welded more easily in proportion to martensite stainless steel types. The most important problem encountered during the welding of ferritic stainless steel types is their extreme tendency against grain growth at temperatures higher than 1150oC. A part of the area influenced by the heat during welding heats up to a temperature level higher than 1150oC and an extreme grain growth takes place in this area. There is no chance for diminishing the grains through a thermal treatment when the austenite contained in this material doesn’t turn into ferrite in solid condition. In the normal condition, ferritic stainless steel has a very fine-grained ductile structure. They crisp when they turn into a coarse-grained structure, notched impact strength decreases and transition temperature increases. Nitrogen is added to the composition of some ferritic stainless steel types to prevent the grain growth. (For instance; according to the AISI norm, maximum 0,035 for ‘444’ quality steel and 0.25 for ‘446’ quality steel).
The nitrogen element added to the electrode helps the welding metal to become fine-grained at the end of hardening process. The welding method applied for welding this stainless steel type must ensure that the welding point in the area influenced by the heat remains at the temperatures higher than 1150oC for the most possible minimum period of time. This can be achieved by applying the welding through very short passes and cooling it immediately. Another problem encountered during the welding of chrome ferritic stainless steel types is the constitution of a very fragile and brittle sigma phase which is an inter-metallic phase of chrome and iron. This happens when the steel parts are exposed to 400 ~ 550oC for a long period of time. So, any preliminary annealing process exceeding 400oC mustn’t be applied to this steel type. However, a preliminary annealing process can be applied at 200 0C. In other situation, no preliminary annealing is applied while these steel types are welded.
One of the dangers encountered during the welding of ferritic chrome stainless steel types is their extreme sensitivity against inter-granular corrosion in the area influenced by welding. This is an important problem encountered especially in the non-stabilized types with high chrome and carbon contents. This incident occurs in ferritic types while they are rapidly cooled down from the temperatures higher than 900oC contrary to the one taking place in austenitic chrome-nickel stainless steel types, this is because chrome carbide precipitation occurs at higher rates in ferritic structures than austenitic structures. Ferritic chrome stainless steel types become sensitive to inter-granular corrosion in the adjacent areas of welding when they are welded. Chrome carbides dissolve earlier and precipitate onto the grain boundaries in lots during the cooling process. Welded joints made of non-stabilized steel with 17% of Cr can be made resistant to inter-granular corrosion by being subjected to annealing at 750oC. If these steel types have been stabilized with Ti or Nb, then welded joints will be resistant to inter-granular corrosion even without applying any thermal treatment.
Any preliminary annealing to be applied during the welding of ferritic chrome stainless steel types will have different metallurgic impacts from the one applied during the welding of martensitic stainless steel types. Welded joints of these steel types result in grain coarsening and toughness decrease when they are cooled gradually. Some ferritic stainless steel types tend to cause martensite constitution on their grain boundaries. The preliminary annealing applied to these steel types eliminates the cracking risk in the area influenced by welding and restricts the stresses resulting from welding process. Preliminary annealing temperature is determined according to the composition, desired mechanical features, thickness and residual stresses. Preliminary annealing is usually applied at 150 – 250oC and interpass temperature can be a little higher.
Applying an annealing process at 750 – 850oC and then a fast cooling process after the welding will help to increase the ductility of the area influenced by welding as well as the resistance against inter-granular corrosion.
18/8 type will work for low carbon ferritic stainless steel types while the electrodes containing 25% Cr and 20% Ni will work for the steel types containing more than 8.1% C. Table 5 and 6 show the chemical compounds and mechanical characteristics of ferritic stainless steel types.
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