Cold Forming Die Casing Material Selection 4

[madojabe]

We are presently taking 5/8 dia. wire and forming it into a parallelogram shape. The die section is split into two pieces and has a six thousandths press fit into a casing that is 5.810 dia. and made from 4340 unheattreated material. We experience some loosening of the die after running approximately one hundred thousand parts with it getting progressivly worse until a new casing has to be used. I was wondering if the 4340 material is the best choice for this and if I might not be better off using the shrink fit method instead of press fitting. The die section must be split due to some finishing, etc. that are easier to accomplish with it being split.
Thanks

 

[metman]

I suggest trying AISI O7 (Type 413) Cold Work die Steel.

Heat treat: 1450-1625F, oil quench, temper at 325-350F.

 

[madojabe]

Thanks Metman. That sounds like a good choice. We work with the air hardening steels so much that I tend to forget what's really out there.

 

[TVP]

H13 tool steel is the most commonly used material for die cases (outer stress rings) in the cold forming industry. It is usually quenched and tempered to 45-50 HRC. 4340 can also be used in the quenched and tempered condition, but it has limited hardenability for thick sections. 4340 that has not been quenched and tempered is not a suitable die case material.

 

[TVP]

madojabe,

What is the outer diameter of the die insert before press fitting?

 

[madojabe]

Again, good info. We used to use a lot of H-13 when we were injection molding so I've got some familiarity with it.
The die section is .900 thk.; 2.316 at the top with a one and a half degree taper to the bottom. The bottom diameter is 2.363. The die is made in two "L" shaped parts following the shape of the paralleogram. The casing has a diameter of 2.310 with the matching one and one half degree taper. It has always seemed to me that it was a lot of interferrence but I was told by the people that initially built the die that that was the right amount due to the tremendous pressures involved in cold forming.

 

[TVP]

madojabe,

Cases and stress rings for cold forming tools typically use a diametrical interference of 0.3 to 0.6% of the interface diameter. In your case, 0.3% would be an interference of 0.0069 inches and 0.6% would be 0.0139 inches; this means that the current 0.006 inch interference is on the low side of the recommended range.

I am trying to calculate the stresses due to the press fit, but I am not sure that I have the geometry exactly right. Is the 0.900" thickness in the same direction as the diameter, or are you describing how "tall" or "deep" the die segments are? What is the length of the 2 parallelogram sides? Once we calculate the stresses due to the press fit, we can add in the working stresses, and see what the total stress will be on the 4340 case. Are the die sections made from a High Speed Steel such as M2?

 

[madojabe]

The .900 is the depth of the die section. The die section material is M2 heat treated to 60-62 R/C and highly polished.
The long leg of the parallelogram is 1.211 and the short leg is .444 with a .375 fillet between them. The other two corners of the parallelogram are sharp, with no radius. The angle that forms the parallelogram is 2 degrees. The finished part thickness is .300.
There is no clearance between the form punch and the die section. Do you think that with the press fits involved, that there might be some reduction of the interior die section dimensions to the point where instead of a line to line fit, there is some interferrence?

 

[TVP]

After reviewing your application, there are two problems with the current setup:

1. The interference fit is too small.
2. The strength of the die case is too low.

The minimum size of inserts and stress rings/case should roughly follow this developed formula:
B (inside diameter of ring/case) = 1.5*A (inside diameter of insert or segment)
C (outside diameter of ring/case = 4.5*A

The ideal ratio is B = 2*A and C = 6*A. Your application is currently B = 1.91*A and C = 4.8*A, so this acceptable. Using the ideal ratios would be an improvement, though.

I would increase the amount of press fit to around 0.008" so that the die segments would be under more compression. And I would quench and temper the die case to be 45-50 HRC. The thickness of the die case is too large to satisfactorily quench 4340 to that type of hardness, so I recommend changing to H13 tool steel. When the press fit is too small, the interface between the segments and the case deforms due to the high forming pressure. I am guessing that the pressure is fairly large in order to fill out the corners of the parallelogram, so either making the assembly stiffer (die segment to case ratio) or stronger (press fit + heat treating) would improve the life of the case. Many die cases are only replaced when the inner diameter has become exceesively worn due to many insertions and removals of the die segments.

 

[madojabe]

Taking your advice and making some new cases and die sections. Thanks for your help.

 

[TVP]

Good luck and let me know how it works out.

 

[madojabe]

Will do.

 

[WhiteTiger]

A bit late, but it might be worth looking at LaSalle ETD 150. The material properties are such that rifle barrels and actions can be machined from it with no heat treat.

 

[Kenneth]

The mechanical properties of LaSalle EDT 150 are substantially lower than the materials typcially sucessfully used (such as H13 suggested by TVP) for die case applcations like this.

For extreme applications in which the expense can be justified, I have ocassionaly seen maraging steels (C250, C300) used for die cases. These materials have an exceptional combination of yield strength (required to retain the insert in compression) and "toughness".