Steels_ Metallurgy and Applications, Third Edition

Stainless steels 337

Austenitic stainless steels

Austenitic stainless steels are readily welded, given their high level of toughness

and freedom from transformation to martensite. Therefore they are not prone to

the cold-cracking problems encountered in martensitic stainless steels and require

neither preheating nor post-weld heat treatment. Because of these characteristics,

austenitic fillers are often used for joining dissimilar steels or, as mentioned

earlier, in the welding of brittle martensitic stainless grades. However, precau-

tions must be taken to avoid other problems in austenitic stainless steels, namely

solidification cracking and sensitization.

Solidification cracking takes place in the weld metal as it is about to solidify.

The problem is due to the generation of high contraction stresses in austenitic

stainless steels because of the high thermal expansion characteristics of these

materials. These stresses pull the solidified crystals apart when they are still

surrounded by thin films of liquid metal, giving rise to interdendritic cracking.

However, it was discovered at an early stage that welds with a fully austenitic

structure were particularly susceptible to solidification cracking and that the

problem could be overcome with the introduction of a small amount of delta

ferrite. The amount of ferrite required to eliminate cracking depends upon the

degree of restraint imposed upon the joint and also on the composition of the

steel. However, a level of about 5% ferrite is generally adequate. Therefore in

autogenous welds, the compositions of commercial grades such as Types 304,

316, 321 and 347 are balanced such that they are free of delta ferrite in the

solution-treated condition but generate about 5% ferrite in the weld metal due

to the thermal excursion into the liquid state. Similarly, in the joining of thick

sections of austenitic stainless steels, filler metals of the correct composition are

employed which develop the required microstructure.

The prediction of the microstructure in the weld metal of stainless steels can

be carried out using diagrams prepared by Schaeffler 21 and De Long) 2 The latter

was developed later and has the advantage that it takes account of the nitrogen

content of the steel. As indicated earlier, nitrogen is a powerful austenite-forming

element and can therefore have a significant effect on the microstructure. The De

Long diagram is shown in Figure 4.21 and the chromium and nickel equivalents

are calculated in the following manner:

Cr equivalent = %Cr + Mo + 1.5Si + 0.5Nb

Ni equivalent = %Ni + 30(2 + 30N + 0.5Mn

Solidification cracking is promoted by the presence of certain elements which

segregate to the remaining liquid during the solidification process, producing

interdendritie films of low melting point. Elements such as nickel, silicon,

sulphur and phosphorus increase the susceptibility to cracking whereas chromium,

nitrogen and manganese reduce the cracking tendency. With this knowledge,

filler metals can be designed which are completely austenitic and yet resistant

to solidification cracking. Such fillers are low in sulphur, phosphorus and silicon

but often contain 7-10% Mn. One particular use of these zero ferrite electrodes

is in the welding of grades such as Type 310 (25% Cr 20% Ni) in applications