TWI Technology Briefing 626 - October 1997
D C Buxton
FULL REPORT
Steels containing approximately 0.5%Cr have recently been developed to provide pipeline materials with increased corrosion resistance in CO 2-containing environments. These environments will frequently also be 'sour', ie contain H 2S, and it is well recognised that ferritic steels in media containing H 2S 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 carbonmanganese steels to 22HRC (approximately equivalent to 250HV).
Background
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 use 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.
Approach
A length of seamless 0.5%Cr steel pipe and a matching welding consumable were obtained. Bead-in-groove test welds were produced by the gas metal arc welding process, using mechanised equipment. In order to obtain a range of heat affected zone (HAZ) hardness values above and below the NACE limit of 22HRC, various arc energies were used. Maximum hardness values were recorded using a Vickers machine with a 5kg load.
Four-point bend test specimens with maximum HAZ hardness values between 237 and 293HV were produced and tested under dead weight loading for a period of 720 hours in NACE TM0177 Method A test solution at ambient temperature. This solution consists of 5% sodium chloride, and 0.5% acetic acid in water, saturated with hydrogen sulphide. The applied outer fibre stress was equal to the measured parent material yield strength of 440 N/mm 2.
After testing, specimens were examined for evidence of cracking by visual examination, dye penetrant testing and sectioning.
Results and discussion
A subsurface crack, with morphology typical of hydrogen induced SCC, was detected in one test specimen. The maximum hardness value recorded on this specimen was 262HV, higher than the NACE limit for sour environments of approximately 250HV. The crack was located on the fusion boundary and had propagated in both the weld metal and HAZ. Cracks were not detected in the specimen with maximum hardness below the NACE limit, or in other test specimens with maximum hardness values of 260 and 293HV.
The 0.5%Cr steel results were compared with available data from a range of carbon and carbon-manganese steels. It was found that the results were consistent. This indicates that existing guidelines for avoiding cracking can be applied to 0.5%Cr materials of the present types, and, in particular, that the NACE MR0175-96 hardness criterion is appropriate.
Locations of the maximum hardness values in the test welds were found to differ. Specimens taken from bead-in-groove welds produced with higher arc energies had the maximum reading located within the weld metal, whereas specimens from lower arc energy welds had maximum readings recorded in the heat affected zone. This is not unexpected, given the different compositions of the weld metal and heat affected zones and hence different hardenability and transformed microstructure.
The maximum hardness levels recorded were compared with values estimated from consideration of the weldment cooling rate and steel composition. The results were similar even though steels of the present composition were not necessarily included in developing the various hardness prediction formulae used.
Conclusions
- A hardness limit of 250HV, equivalent to 22HRC, stipulated by NACE MR0175-96 is applicable to welded 0.5%Cr steel.
- Welds in steel containing 0.5%Cr show similar hydrogen-induced stress corrosion cracking behaviour to carbon-manganese steel welds.
- The HAZ hardening behaviour of the 0.5%Cr steel can be described using predictive systems derived for C-Mn steels, although these may be somewhat conservative.
Recommendations
The maximum hardness level in welded 0.5%Cr steel pipe, when used in sour environments, should be limited to 22HRC (250HV) in accordance with NACE MR0175-96.
Member Report No. 626-1997
Behaviour of 0.5%Cr steel in sour environments
