Welding of Stainless Steel
Steel is made stainless by the addition of alloying elements such as chromium, nickel and molybdenum.
The characteristics of a stainless steel is it's ability to withstand attack from the atmosphere, water and from many solutions of acids, salts and alkalis.
This resistance is achieved by alloying the steel, primarily with chromium. Other alloying elements are manganese, titanium, niobium, copperand nitrogen.
Among the stainless steels, the austenitic types in particular exhibit very good weldability. These are alloyed with both chromium and nickel, although other elements are sometimes included.
DIFFERENT ALLOYING ELEMENTS GIVE RISE TO DIFFERENT STRUCTURES
As it solidifies, a steel crystallises into a metallographic pattern determined by the alloying elements it contains. In the case of stainless steels, the pattern or structure maybe one of the types known as austenitic, ferritic or martensitic.
The relationship between alloying element and structure may be obtained from the Schaeffler-De Long diagram, which is divided into zones according to the differing structures of stainless steel. The coordinates are graduated in parameters known as the "chromium equivalent" and the "nickel equivalent".
The chromium equivalent is based on the percentage of chromium and all other alloying elements which have the same effect as chromium on the structure, e.g. molybdenum and silicon. These are all ferrite formers. The nickel equivalent is derived in a similar manner. It is based on the percentages of the austenite formers among which nickel, manganese, carbon and nitrogen are included.
The majority of the chromium nickel steels are on the borderline between the austenitic and austenitic ferrltlc zones.
FERRITE NUMBER
The amount of ferrite in steel is generally expressed in terms of it's ferritic number FN. For the welding of austenitic stainless steel a ferrite content of 2-12 FN is required. A ferrite content exceeding 12 FN can lead to a continuos network of ferrite increasing the risk of sigma phase formation which reducesthe corrosion resistance and toughness. Excessively low ferrite content i ncreases the risk of hot cracking.
FACTORS TO CONSIDER WHEN CHOOSING AN ELECTRODE
The electrode should be of the same basic analysis as the parent metal. This gives the weld it's optimum corrosion resistence. However, certain exceptions are permissible, subject to expert approval. A high?alloy electrode maysometimes be used for welding a low alloy parent metal. Thus, for example, 304 stainless steel with molybdenum?alloyed electrodes. This gives satisfactory results as regards weldability and mechanical strength. However, the corrosion conditions must be considered. Thus, in citric acid, grade 304L is more resistant than 316L.
In such applications, grade 304L should be welded with 308L/MVR electrodes and notwith any of the higher alloyed electrodes.
The nitrogen-alloyed steels may be welded with the equivalent grade of electrode, which doesn't have to be nitrogen?alloyed since the weld metal will reach a sufficiently 0.2% proof stress by absorbing nitrogen from the parent metal. However, nitrogen alloyed electrodes should be used if the component is to be annealed after welding or if design calculations are based on ultimate tensile strength.
