Effect of Grain Structure on Joint Strength

By clshamie Date 04-04-2007 21:25

I have a general question related to welding:

For base steel of a given carbon content (specifically, 1035) will the hardness of the steel effect the strength / quality of a weld applied to it? In other words, if I keep the carbon content the same but increase the hardness by changing the crystalline structure of the metal through heat treatment, will I run into welding issues? My first instinct is that the increased hardness will have no effect as brittle welds result from untempered, brittle martensite formation when carbon content is high. But if carbon levels do not change, the weld integrity should not change either.

Any thoughts?

Thanks,
Chris

By GRoberts (***)

Date 04-04-2007 23:06

In some ways you are right that you will not increase the welding problems by increasing the hardness, in that the weld will have the same composition, toughness, etc. However, the harder will make the weld possibly more difficult in other ways. The stronger material will provide higher joint restaint and place more residual stresses on the weld. There is also more chance of hydrogen cracking in the base metal due to a more susceptable microstructure.

By 803056

Date 04-05-2007 02:54

In general, the weldability of carbon steel is considered to be good if the carbon content is held to 0.3% or less. If the material is cold worked or otherwise processed to increase the hardness, the tendency to initiate cracks is increased. Joints that involve intersecting welds can be more troublesome and prone to cracking because the forces developed by the residual stresses are added as vectors. The resulting force can exceed the tensile strength of the base metal. If the base metal is hard to begin with, i.e., has low ductility, the tendency to crack is greater than if the same base metal was not "hard". The thickness and the restrain of the joint must also be considered.

The cooling rate must be considered. Rapid cooling is going to result in harder weld deposits and heat affected zones. To minimize the potential to "harden" the weld and HAZ, preheat is employed.

A piece of steel with lower carbon content that has been cold worked or austenized and quenched can be harder and less ductile than a piece of steel with slightly higher carbon that has been annealed. How hard the steel can become is a function of the total alloy content, i.e., carbon equivalency, and the cooling rate from the austenizing temperatures, i.e., as the weld solidifies and as the weld and HAZ cool, if heat treatment is the only variable.

Best regards - Al

By clshamie Date 04-05-2007 11:34

Thanks Guys.

By js55

Date 04-05-2007 14:02

You don't really change the crystalline microstructure of carbon steels through stress relief type PWHT. What you do is 1) essentially relieve stresses that create triaxial zones of little 'give' by actually taking them beyond their yield point due to the high temp regime, and 2) draw carbon out of solid solution (the primary strengthening mechanism of carbon steel) by forming carbides. As stresses are relieved and carbides formed hardness testing will generally register a diminishing (I say generally because sometimes the manufacturer has added elements such a Boron or Vanadium for added strength to assist in strengthening without adding more carbon, which of course contributes to crack sensitivity, and these microelements, that often do not need to be reported, are responsible for hardening precipitations depending upon time/temp/volume% relationships). Unless of course you do microhardness right on top of a carbide.