TWI Industrial Member Report Summary 626/1997
By D C Buxton
Background
Steels containing approximately 0.5%Cr have recently been developed to provide pipeline materials with increased corrosion resistance in C02-containing environments. These environments will frequently also be "sour", i.e. contain H2S, and it is well recognised that ferritic steels employed in media containing H2S are susceptible to hydrogen induced stress corrosion cracking. To avoid cracking in practice, hardness limits are stipulated. NACE standard MR0175-96 limits the maximum hardness of carbon and carbon-manganese steels to 22HRC (approximately equivalent to 250HV). The standard was derived originally for parent steels, but recognises the use of welding for fabrication. During fusion welding, local hardened microstructures sensitive to embrittlement may be formed. It has been confirmed that this maximum hardness limit is applicable to welded joints in carbon and carbon-manganese steels, but adequate data are required for safe utilisation of weldments in new alloys such as 0.5%Cr steel.
Objective
To determine if the NACE hardness maximum for avoidance of sulphide stress corrosion cracking is applicable to weldments in 0.5%Cr steel.