4140 Specs Do Not Define Mechanical Properties !!? 3

[tc7]

I have to prepare a new weld procedure based on AMS 4140 normalized material. The specifications I have been given is either AMS-S-5626, AMS 6382 or AMS 6349. None of the specifcations tell me what the expected UTS or hardness is for materials produced to these specifications. Will need to know elongation as well. The subsidiary reference specs listed therein don't give me any clues either. Certainly there are handbooks and web sites that will give ranges of hardness for this alloy, but these seperate sources of information are not binding when i cite the above AMS specs in my purchase order. What is the secret document that I am missing?

 

[TVP]

You are not missing any secret documents. Standards that have been developed in North America (SAE, ASTM, etc.) are notable for their lack of mechanical property requirements. Standards committees for products like steel sheet and bars have significant representation by producers of steel sheet and bars and apparently they favor no mechanical property requirements. It's ridiculous in my opinion, but I'm just a freak on the internet...

 

[KirbyWan]

I believe that MMPDS-01 has mechanical property data on this material. It's free and online, somewhere.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[metengr]

TVP;
I would have to respectfully disagree with you with mentioning ASTM . Most of the material specifications for ferrous and nonferrous materials cited by ASTM have minimum stated mechanical property requirements for Grades or Classes of materials based on intended service or applications.

 

[CoryPad]

metengr,

I am not sure if "most" ASTM standards have mechanical properties, but there are several that don't, which doesn't make a lot of sense to me. Examples include:

ASTM A 108
ASTM A 109
ASTM A 322
ASTM A 434
ASTM A 576
ASTM A 659
ASTM A 794

 

[MikeMet]

The reason they don't mention mechanical properties for AISI 4140 is that it is not commonly used as a finished part in the normalized conditon, it is a heat treatable alloy. In the normalized condition you can have a mixed phase microstructure that depending on thickness and cooling rates (which are not specified in the standards) or if you subcritical cool after normalizing. All of which can give you non-homogenous properties across the thickness. Hard to design for using non-homogenous properties. You will have significantly different properties in the weld and weld HAZ in the as-welded condition and if you re-normalize the part you will change the properties of the base metal slightly.

 

[tc7]

Can anyone take a shot at reasonable or permissible through thickness hardness variation? In 2-inch thick barstock (cut from 10-ft lengths) I get readings from Rockwell B97 on the outsides (mill scale ground off) to Rockwell B88 at dead center. This seems like a wide spread but I have no idea what I should expect (again, the material is 4140 Normalized).

 

[CoryPad]

Specifications for steel bars in the normalized condition can have wide ranges, like 170 to 230 HB (87 to 98 HRB). This wide range is to accomodate variations in chemical composition, microstructure and residual stress produced via thermomechanical processing for series production. The fact that you see this much variation within a single part is not unlikely, but it does show that you have large variability.

 

[metengr]

tc7;
Request a mill test report from your supplier. This will provide you the necessary information regarding your WPS qualification.

 

[tc7]

General follow-up question......
Can we expect colled rolled 4140 to be more consistent as far as through thickness hardness variation?

 

[MikeMet]

Cold rolling before normalizing may help a bit, but again it comes down to thickness and cooling method after normalizing. I think the hardness difference you are seeing is not unexpected. If the bar, even 2" bar, is still air cooled the cooling rate on the surface will be much faster than the center. Therefore, the higher hardness on the surface. If cooled in agitated air the difference may be even more. I suggest a normalize and temper after completion of all welding. There are numerous ASM books that show expected strength and elongation at various tempering temperatures. But those are averages and what you get is very dependent on chemistry of you particular bar.

FYI:
Properties of 4140 50mm bar normalized, air cooled:

http://www.matweb.com/search/DataSheet.aspx?MatGUID=d9811a8822114ea68653e00f1e641ce9

Properties of 4140 50mm bar normalized, air cooled, tempered at 1000F :

http://www.matweb.com/search/DataSheet.aspx?MatGUID=1897ec2242d14abba04ef1de7e7be01d