Stainless steels play an essential role in all sectors of the oil and gas industry, both upstream and downstream. In the past, this has predominantly entailed established grades, but the last 10 to 20 years have seen major changes in the design and use of stainless steels for oil and gas service. A range of low carbon 13%Cr martensitic, highly alloyed austenitic and two phase ferritic-austenitic steels has been developed offering improved properties and cost effectiveness for a wide range of duties, and the materials are being employed in increasing tonnages. However, the practical application of such steels is critically dependent on the use of welding for fabrication, since a welding operation can have a significant influence on the corrosion performance of the completed assembly.
The paper considers the welding behaviour of the new steel types relative to older grades. Potential problem areas are identified, with the aim of indicating the controls necessary so that full advantage can be taken of the attractive base metal properties.
The diverse conditions experienced in oil and gas plant require close attention to material selection in order that the most cost effective and reliable materials can be employed as appropriate. This has led to a high level of awareness of the properties offered by stainless steels, and in consequence these materials play a major role in both upstream and downstream sectors of the oil and gas industry. In large part, they are employed in plant and associated equipment where the corrosion resistance of plain carbon or low alloy steels is inadequate, although the austenitic grades in particular find applications where their excellent elevated temperature or cryogenic mechanical properties are of advantage.
Much oil and gas technology is mature practice and thus use of stainless steels mainly entails well standardised grades, with austenitic and martensitic alloys being dominant and, historically, with less utilisation of ferritic alloys. However, the last 20 years or so have seen development of new alloys which differ appreciably from traditional materials in composition and microstructure, and these steels are being increasingly specified for oil and gas service. The present paper considers the welding characteristics of stainless steels of significance to the oil and gas industry, with an emphasis on the newer materials.
MARTENSITIC STAINLESS STEELS
Established Grades
Welding procedures for hardenable 13%Cr steels are designed primarily to avoid hydrogen fabrication cracking and to obtain adequate weld area toughness (~). This normally involves both preheat prior to welding and postweld heat treatment (PWHT). Given appropriate control of temperature at all stages, it is a fairly straightforward operation to obtain sound joints in plain 13%Cr alloys giving the desired mechanical properties. Particularly stringent attention is necessary with the higher carbon grades such as AISI 420, but welding can nevertheless be carried out reliably to meet appropriate specification requirements.
For sour service, NACE MR0175 specifies maximum hardness levels for base metals, and requires that welded joints should meet similar hardness limits. There has been very little attention to determining the SSC behaviour of weldments in 13%Cr steels, and some reduction in weld metal behaviour might be expected, recognising the inevitability of alloy element segregation during solidification and of a high inclusion population. Nonetheless, compliance with base metal hardness limits has normally yielded satisfactory service of welded joints. For martensitic stainless steels, a postweld tempering operation is necessary to
