H13 equalized too long 1

[smslab]

What are the consequences of equalizing H13 too long when ramping up to the austenitizing temperature? Standard procedure is to heat to 700 C, equalize for 15 min, then up to 850 C, equalize for 15 min, and then ramp up to the austenitizing temperature of 1030 C. Thanks to some problems with the furnace, the equalizing time at 700 C was 30 min, and the time at 850 C was 7 hr. The rest of the heat treat proceeded according to the specification.

Is this likely to have an effect on the properties of the H13? Should the heat treat be repeated?

 

[stanislasdz]

smslab

The time of equalizing depends on the thickness of the parts (rule of thumb) around 1 hour per inch

Too long equalizing time at 850 °C lead to abnormal grain growth. So you risk to deteriorate the mechanical properties.

Too long time at 700 °C presents no negative effects- I guess- on the mechanical properties.

 

[metman]

I don't think tensile properties will be reduced rather possibly might be increased but toughness will be adversely affected by the increased grain size. Since the recommended austenitizing temperature is 1862F, considerably above the equalizing temperature of 850C(1562F), and since temperature has more effect on grain growth than time, it might be that the abnormal grain growth is marginal as affecting properties depending upon application requirements and spec's. However, 7 hrs is a long time so, abnormal in the usual usage of the terminology is probably a correct moniker.

What is the application? Is there impact loading or fatigue loading? What are the required spec's? What is the economic fallout -- one expensive tool, many tools, relatively expendable die?

A simple re-heat treat will not correct the problem. Although air hardening tool steels are not usually normalized, it might be effected in this instance for the purpose of refining the grain size and making it more amenable to re-heat treatment. I am not suggesting this is the answer rather posing the question to those more competent to comment. If there are no formal spec's to be met but instead an inhouse useage with no impact or fatigue loading, could it be used as is?

I was just reading in Third Edition of Tool Steels by Roberts/Hamaker/Johnson (1962) that (H11)type 520 had fatigue values reported to be the highest of any known engineering material. Now we have maraging steels that are at least in this fatigue strength/UTS range or maybe greater.

 

[smslab]

Thanks for the replies. I'm not at liberty to name the application, but it involves thermal cycling and limited impact loading. The inhouse spec is vague - it calls out a hardness but does not for example disallow any of the microstructures that NADCA 207 disallows.

What do you think of this though? I found some data for H13 on MatWeb (if you want to see, search for "Latrobe H13 Tool Steel (ASTM H13)" on MatWeb) that say that Ac1 is 840 C and Ac3 is 890 C. I would think that there would not be any significant grain growth unless the steel was in the single phase austenite field.

 

[redpicker]

I don't think that 7 hours at 1560 F would contribute much to grain growth, but it would be possible. 7 hours is a long time, but I would think H13 to be rather resistant to grain coarsening, so I wouldn't be that concerned.

I'd be more concerned about decarburization/carbuirzation. What kind of furance atmosphere control was there? Did the "problems with the furnace" affect this control?

rp

 

[metengr]

smslab;
If I was the customer, the obvious answer to this question is to evaluate a sample of the H13 material that was subjected to the nonconforming heat treatment. The evaluation could be as simple as hardness testing and metallographic examination of the microstructure. This way you have no speculation and the customer is satisfied.

 

[smslab]

They've done hardness and not suprisingly it was fine. I have advised them to check grain size.

 

[Maui]

What is the physical size and geometry of the part? Was the heating performed in a salt bath, or an atmosphere controlled furnace, or some other furnace design? What are the requirements that are called out in the customer specifications for this component? These are questions that will need to be answered in order to provide you with a worthwhile response.

I agree that a metallurgical evaluation may be prudent, and probably needs to be performed in order to verify compliance with the customer specifications, whatever they are.

Maui