Steels_ Metallurgy and Applications, Third Edition

336 Steels: Metallurgy and Applications

mechanical properties similar to that of parent material. Therefore good weld-

ability is a particularly attractive feature of stainless steels and this contributes

very significantly to the wide usage and versatility of these materials. The welding

of stainless steels has been reviewed extensively by Castro and de Cadenet 19 and

the topic has been updated more recently by Gooch. 2~

Martensitic stainless steels

Because of their high alloy content, 12% Cr steels and other martensitic stainless
grades exhibit a high level of hardenability and are capable of transforming to
martensite in large section sizes. Therefore these steels are susceptible to cold
cracking in the weld metal and HAZ in a similar manner to low-alloy marten-
sific grades. As with their low-alloy counterparts, the problem is exacerbated
by the presence of hydrogen, and the risk of cracking increases as the carbon
content and hardness of the steels are increased. For this reason, the welding
of 12% Cr martensitic grades is generally restricted to compositions containing
a maximum of about 0.25% C. However, the problem of hydrogen cracking
is well understood and can be readily overcome in martensitic stainless steels
provided the normal precautions are taken. Thus in gas-shielded processes, pure
argon should be used rather than argon-hydrogen mixtures, and in manual metal
arc welding, low-hydrogen basic electrodes should be employed. The thorough
baking of electrodes is also recommended in order to remove the last traces of
moisture.
Like other highly hardenable grades, 12% Cr steels are generally preheated to
reduce the risk of cracking. Preheating temperatures of up to 250~ are employed
which ensure that the weld metal and HAZ cool slowly through the Ms-Mf
temperature range, thereby facilitating stress relaxation and reducing thermal
stresses. On the other hand, martensitic stainless steel containing less than 0.1% C
and with nickel additions of up to 4% are often welded without preheat. However,
it is still imperative that low levels of hydrogen are produced in the weld metal
with matching fillers so as to avoid weld metal cracking. The use of Cr-Ni
austenitic filler metal can also remove the need for preheating, although the
strength of the joint will be lower than that achieved with martensitic weld
metals.
After welding, 12% Cr steels are subjected to heat treatment in order to provide
an adequate balance between strength and toughness in the weld zone. Because
these steels have a low Ms-Mr temperature range, austenite may well be present
immediately after the completion of welding and it is essential that the assembly
is cooled to room temperature in order to complete the transformation before
applying post-weld heat treatment. If adequate cooling is not carried out, austenite
will be present in the microstructure during the subsequent tempering treatment
and this may well transform to untempered martensite on cooling to room temper-
ature. Post-weld heat treatments are generally carried out in the range 600-750" C.
Martensitic stainless steels can be welded using all the conventional fusion
processes, providing precautions are taken to minimize the level of hydrogen in
the weld.