Stainless steel is a type of high alloy steel, usually containing elements such as nickel (Ni), chromium (Cr) and molybdenum (Mo) in its chemical composition. The presence of these elements, especially chromium, gives stainless steel an excellent corrosion resistance over carbon steel. To be classified as stainless, the steel must contain at least 10.5% chromium, and can be classified into three types according to the formed microstructure: Austenitic, Ferritic or Martensitic.
Stainless steels can be divided between two large groups, according to their microstructure. The specific microstructure is reached according to the alloying elements, following those that stabilize the ferrite and those that stabilize the austenite, as below:
Elements that stabilize the ferrite: Cr, Si, Mo, Ti and Nb;
Elements that stabilize austenite: Ni, C, N and M.
The chemical composition of the stainless steel along with the thermo-mechanical processing gives them different properties. In this way, each group of stainless steel is indicated for different types of applications. Check below the types of stainless steel and the application of each one of them:
Austenitic Stainless Steel
Main characteristic: resistance to corrosion.
Application: Equipment for food, pharmaceutical, chemical and petrochemical industries, civil construction, dishes and other household items.
Stainless Steel Ferritic
Main feature: corrosion resistance and more affordable cost.
Application: appliances (microwaves, refrigerators, stoves, among others), refrigerated counters, coins, cutlery and automobile industry.
Martensitic Stainless Steel
Main characteristic: high hardness.
Application: Surgical instruments, cutting knives, brake discs and cutlery.
It should also be noted that in addition to these classifications, there are also the so-called duplex stainless steels, which have 50% ferrite and 50% austenite and the precipitation hardenable stainless steels.
In welding processes, even simple materials such as carbon steel require a rigorous analysis of parameters to achieve stability. In the case of welding of stainless steels the complexity/difficulty is much greater. Due to the wide range of possible alloying elements and contents of these elements in different stainless alloys the parameterization becomes very complex.
Challenges such as excessive grain growth, formation of cracks during solidification, cold cracking induced by hydrogen and precipitation of unwanted phases must be overcome with the correct parameterization for each type of alloy.
In order to relate the chemical composition of the stainless steel with the microstructure to be obtained, Schaeffler developed in the 1950s a diagram divided into four regions of chemical composition, showing some types of discontinuities found in the welding of this material. The regions are as follows: grain growth; cold cracking induced by hydrogen; precipitation of sigma phase between 600 and 950 ° C and solidification and liquation crack. Among these there is a fifth region, located around 21% Cr and 10% Ni and free from any kind of problem.
To use the Schaeffler diagram, we have to calculate the equivalent chromium and nickel of the materials used with the equations:
The percentages of each element represent the performance of the element as a gamma/alfagenic element when compared to the effects of nickel and chromium, depending on the group in which the element is.
The phases presented by the diagram are austenite, ferrite, martensite, as well as combinations of two and even three phases. As previously mentioned, each phase has a greater tendency to present one type of defect. Thus, it is possible to estimate before starting to weld the expected final defects and, therefore, to employ corrective actions against them:
Sensitive to grain growth. Ferritic inox may show irreversible grains growth for long stay at temperatures above 1150 °C.
Material susceptible to hot cracking. This type of defect, related to the formation of solidification and liquation cracks, is usually associated with impurities of the material, such as S and P. Austenitic stainless steel are especially sensitive to this defect because there is low solubility of sulfur in the characteristic centered cubic matrix of the austenitic phase. To reduce the susceptibility to this defect, the correct addition material must be chosen. It must guarantee the chemical composition of the welding to lay in the austenite-ferrite region with ferrite content from 4 to 10% maximum. Thus, the formed ferrite can dissolve some of these impurities and relieves residual stresses during the cooling, decreasing the cracking occurrence. Another measure is to act on the chemical composition of stainless steel, limiting the contents of P and S to a maximum value of 0.04%.
Region of martensite (or martensite + α / martensite + γ)
The martensitic phase has great susceptibility to cold cracking (or cracking by hydrogen), due to its characteristic brittleness.
The chemical composition of stainless steels in this region favors the appearance of a new phase in the steel, known as the sigma phase. It is basically composed of iron and chromium and has great fragility at room temperature. It is formed after long stay at temperatures between 500 and 900 °C.
Free from the four previous defects. Thus, it is recommended that specific weld material be used so that, upon dilution with the base material, the final composition of the solder is in that region of the Schaeffler diagram.
The Schaeffler Diagram, while being of extreme assistance to the welding procedures for stainless steels, does not consider nitrogen as a gama former element. This factor was added in the 70’s by DeLong, modifying somewhat the formula of nickel equivalent. This new diagram, known as DeLong Diagram, can also be used for the prognosis of welding stainless steels.
Corrosion in certain environments occurs even in the most resistant stainless metals. Its occurrence and analysis is not addressed by the Schaeffler diagram. It is necessary to take care of the shape of the weld bead, which should not be too irregular to avoid accumulation of dirt and consequent corrosion. In addition, ferritic and austenitic stainless steels are subject to a special type of corrosion called sensitization. It is caused by the precipitation of chromium carbides in the grain contours, making adjacent regions poor in chromium. As this element increases the corrosion resistance of the material, this region becomes more sensitive to the phenomenon and there is an intergranular fracture of the material, that is, along the grain boundaries: