HIC Material 2

[picasa]

Some times for sour services, HIC steel is preferred. In some specifications, simulated post weld heat treatment cycles need to be performed for a certain steel to qualify as HIC steel. For example, three simulated PWHT cycles need to be performed on a test coupon before carrying out detailed chemical analysis and mechanical testing of that coupon. Temperature need to be maintained at 1150°F for a total of three (3) hours.

Question: What happens to the steel when it is held at 1150°F for a long period of time. Why is such a steel which after such simulated PWHT meets certain chemical and mechanical testing criteria, good for sour service?

 

[metengr]

Your question really relates to the effect of post weld heat treatment on material properties (strength and toughness) and HIC resistance of SA-516 plate, as an example.

In general, to enhance HIC-resistance, plate steels like SA-516 are specified to have a lower sulfur content and are heat treated. The heat treatment can be quenched and tempered (Q&T) or normalized (N).

During fabrication of vessels, the application of heat from welding locally alters the original heat treatment condition of the plate (Q&T or N). In some cases, post weld heat treatment (PWHT) is required to reduce residual stress and to decrease the hardness of the base metal heat affected zone from welding, thus reducing susceptibility to in-service cracking ( or HIC). Extensive testing is performed after several or one long PWHT cycle to evaluate the reduction in mechanical properties and HIC-resistance of the plate base material.

If you want more information on this topic, the web site below contains information on HIC-resistant plate steels;

http://www.intlsteel.com/content/products/prod_plate_lit.aspx

 

[weldtek]

Metengr,
Excellent link. Thanks,

 

[SJones]

The hardness reduction is principally aimed at sulphide stress cracking resistance (SSC) for the welds rather than HIC resistance of the plate as HIC will occur anywhere in a susceptible material and not just the weld zone that may or may not have been affected by heating. There is another cracking mechanism, SOHIC (stress oriented HIC)that could be impacted positively by the stress relief. However, I would say that the test regime is to ensure that mechanical properties are maintained under a possible scenario involving 3 PWHT treatments as may occur with repairs.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/