Martensite in HAZ... 1

[PAN]

Please advise me in the following items.

1. Is it acceptable if we can find martensite in heat affected zone of high tensile strength (carbon steel)? The microstructure is checked by replica.

2. If we would like to confirm hardness...What is the maximum hardness which is acceptable?

3. If the hardness is too high...What should we do for correction?

Thanks for all comments in advance.

 

[Weldan]

1. Martensite in the HAZ. Several high strength steels are comprised of a partially martensitic microstructure. If this is the case there could most certainly be evidence of martensite in the HAZ. As far as it being acceptable, this really depends on your application and your own (or your customers) standards for acceptance.

2. Hardness. Once again, this depends on your own standards. Rockwell hardness would be a good choice for determining the coarse hardness of the area (weld).

3. If hardness is too high, there are ways to reduce this. Tempering of the weld or controlling the cooling rate are ways to reduce the hardness.

 

[nousakmj]

1. The presence of martensite is not necessarily detrimental unless, of course, you're concerned about ductility or impact properties. Your application and the strength level you seek in the weld deposit will determine it's acceptability.
2. Since tensile strength and hardness are directly correlated, you should be able to figure out a minimum and maximum value based on the strength of the base material. Testing with a Rockwell type unit will give you a gross representation of the hardness in the area. The accepted method, however, for testing heat-afffected-zone hardness values is by using a Vickers type testing unit with a 10 kg load. Some people use a Knoop indenter, but the standard Vickers pyramid is better.
3. If it's too hard, then postweld heat treatment (PWHT) is appropriate. Make sure that the PWHT temperature is below the tempering or stress relieving temperature of the base material.

 

[arunmrao]

If the martensite transformation has occurred in the HAZ during welding,it is likely to have an effect on the impact properties and in a machined component on its dimensional stability. It would best be advised to reduce this atleast by adopting a post weld tempering cycle.

 

[welding1]

Martensite is formed when the atomic structures changes and has to eject carbon. If the change occurs too quick the structure becomes distorted making the material hard and brittle but martensitic grades of stainless steel can be welded.

To reduce the problem:-

1 Slow the cooling rate by using pre heat and allow to cool slowly

2 Reduce the carbon content. ( other steel ingredients like Chrome will slow the transformation down so that slow cooling will have the same effect as quenching, but carbon is still the main factor in hardening steel) most modern high strength weld able steels achieve great strength by pinning the structure with small atoms such as boron rather than using the carbon transformation.

3 Temper the steel by post weld heat treatment.

It depends on the application some standards may not require PWHT up to a certain thickness.

The main problem with martensite in welds is the risk of hydrogen cracking, see

http://www.gowelding.com

for further details.