Does The HAZ Have a Sharply Defined Boundary? 1

[tc7]

The attached photo is a closeup of a macroetch specimen where we applied a bed layer (two passes) in a .125" deep groove then overlayed a .125" thick layer (two passes) of ECoCr-A surfacing. This is part of a welder qualification test. My question pertains to the darkest grey band below the bed layer. I would guess that this is a demonstration of how deep the HAZ will extend by the weld procedure used.

If I am right and this band is a portrayal of the HAZ, I would like to know why the HAZ has such a sharply defined boundary? I was always of the idea that HAZ was a gradually diminishing area that was related to the varying temperature differential. And why is this band darker than the unaffected base metal?

Please explain for the curious and thankyou very much.

 

[tc7]

I attached another view of another specimen, hoping that the darker zone shows up more vividly.

 

[GRoberts]

What you are looking at is definately the HAZ. Sometimes teh HAZ will be gradually diminished, as you describe, and sometimes not, as you seen in your pictures. It just depends on the steel/etchant used. I am presuming that the sharp lines are the edge where either the upper or lower transformation temperature was achieved, so the microstructure was modified.

 

[metengr]

tc7;
This is a typical HAZ appearance after macroetching. The sharpness or boundary of the HAZ from macroetching is a function of the welding process used, preheat, base metal composition and etchant, as mentioned by GRoberts.

 

[TVP]

Another thing to keep in mind is that the boundary will look different as you increase the magnification. On a micro level there will be less of a distinct boundary, as the microstructure changes. Performing a microhardness survey across the weld will also demonstrate this. You should polish the sample after grinding in order to obtain a suitable surface for higher magnification viewing.

 

[tc7]

The base metal in these specimens is 9310 (AMS 6260 and AMS 6265). It has the following composition:
C .07 - .13
Mg .40 - .70
Si .15 - .35
Cr 1.00-1.40
Ni 3.00-3.50
Mo .15 max
B 10ppm
Cu .35 max

How would you expect the 9310 HAZ to behave? Will it develop a hardened zone? that can be annealed by a PWHT at 1100 deg F for an hour? The reason I ask, is that the carbon content seems so low, like mild steel, that maybe this material won’t develop a hardened zone.
What role does the high nickel and chromium play in developing hardened weld affected areas?
Thanks for helping.

 

[metengr]

tc7;
The hardness of the HAZ will be affected by carbon content; the lower the carbon, the lower the hardness. The depth or width of the HAZ will be affected by alloy elements that increase hardenability; like Ni and Cr.

To answer your question, the HAZ will have martensite and some bainite, as a result of the Ni and Cr alloy additions. Tempering the HAZ, not annealing, will result in tempered martensite and bainite with reduced residual stress - all good things to improve toughness.

 

[GRoberts]

Giving a weld a PWHT will typically also reduce the visibility of the HAZ in macro's such as the ones you posted. However, I am not aware of any correlation between carbon content and/or hardness and HAZ visibility, except that the different constituents of the HAZ, such as martensite, that the base metal does not contain will etch differently.

 

[metengr]

GRoberts;
The hardness of martensite and bainite formed in a HAZ or in air hardenable steels (it really does not matter) is directly proportional to carbon content. You can obtain this information from either ASME Handbooks or any metallurgical engineering text book

 

[GRoberts]

Sorry if I was mis-communicating. I realize maximum HAZ and carbon content are proportional. I was trying to say that I was not aware of anything relating hardness to etching potential or differences. Once you have martensite, I don't know that soft martensite etches any differently than harder martensite. Once you are at the etch stage though, it is easy to do a few hardness checks.