Hardness and Impact Testing

[Angsi]

Ok, I understand hardness testing is not reaally a Code requirement, but is a measure of weld quality and is the industry (oil and gas) norm these days.

Re charpy impact testing. I have also come across requirements where impact testing is required for some weld procedures (regardless of its MDMT), especially on carbon steel and duplex ss materials. Is it also used as a "general measure" of the weld quality?

Thanks.

 

[metengr]

No, impact and hardness testing really have nothing to do with actual weld quality. These are supplemental tests to evaluate the extent of tempering (hardness) and toughness (Charpy impact) of the weld, HAZ and base material after welding, or to assure adequacy of heat treatment. Certain materials can become embrittled if proper precautions are not followed either during heat treatment or during welding.

Impact and hardness testing are really methods used to assure process parameters specified during fabrication have not resulted in adverse metallurgical properties.

 

[Angsi]

I have noticed hardness testing being specified for all (CS, SS, DSS) materials but the impact testings only for CS and DSS. Any reasons you are aware of metengr?

Thanks.

 

[metengr]

Hardness testing is indeed specified in certain applications of materials that require a maximum hardness to avoid susceptibility to SSC or SOHIC in petrochemical service environments. I certainly have not seen hardness testing specified for all CS,SS, DSS applications in the Power Generation industry.

Impact testing is really done for two reasons – to assure a minimum level of toughness for low temperature service applications of materials, and to assure that during fabrication (or welding), the material toughness does not fall below a certain minimum design metal temperature toughness specified by the code of construction.

 

[metengr]

Just to add - ferritic materials will see a reduction in toughness as the service temperature is decreased, which is called the "ductile-to-brittle" transition temperature. The key is to define this transition temperature using impact testing and make sure you remain above this temperature in service.

Examples of ferritic materials are carbon and low alloy steels, ferritic stainless steel - duplex stainless steels are unique because they contain a mixture of austenite and ferrite phases. Duplex stainless steels can also exhibit low toughness behavior in exposure to low temperature service or if they were not properly solution annealed.

 

[davefitz]

Hardness testing is also being recommended , and being proposed as an ASME code requirement, for the advanced high temperature creep resistant ferritic alloys, such as P91, P92, P112. This is to ensure proper sequence of heat treatment and martensite foromation , following bending or welding, and following N+T ( to ensure adequate fast cool down rates were observed).