Question on Steel Grain Size 3

[SuffolkVAEngineer]

Hello All:

Is there a standard specification (ASTM) which details the grain size of a given steel material. As an example, I have specified an forged 8620H steel for some gearing. My heat treater has noted that post-heat treat microscopic analysis of a sample yeilded a grain size greater than he expected for the material. The question presented to me, and hence to you - the members of this forum, is what is the grain size supposed to be in a green state? My thought was that this answer should be found in a standard but I have yet found any specific information regarding this.

Any help that ya'll can provide is greatly appreciated.

Best Regards

 

[TVP]

It depends on how you and your suppliers specify the raw material and the intermediate product forms. For example, if the original hot-rolled bar is ordered according to ASTM A29, then Section 5. Grain Size Requirements is applicable. Here is an excerpt:

When a fine austenitic grain size is specified, the steel shall have a grain size number of 5 or higher as determined in accordance with Test Methods E112. Conformance to this grain size of 70 % of the area examined shall constitute the basis of acceptance. One test per heat shall be made unless the provisions of 5.1.2.1 or 5.1.2.2 are exercised.

If you are specifying steel forgings according to ASTM A788, then grain size is covered in the supplementary requirements section S15 Grain Size.

 

[redpicker]

I am a bit confused by your question, "What is the grain size supposed to be in a green state", when you have earlier stated "post-heat treat microscopic analysis of a somple yielded a grain size greater than he expected for the material". Are you considering "post-heat treat" to be "green"?

The grain size in the "green" state, or prior to any heat treatment is generally not very important. In some specialized cases, controlling distortion during carbuirzing specifically, this can be a concern. Is that your concern, here? It is not unusual for the "as-forged" or "as-rolled" grain size of a low-alloy steel such as 8620 to be rather large, but as these steels are nearly universally aluminum-killed, they will develop a fine grain druing heat treatment. Of course, high-temperature carburizing (above, say 1800F or so) can result in a large grain size after carburizing, but these materials are typcially given a quench and temper treatment after carburizing to refine the grain.

A "grain size greater than he expected" is also not very specific. What GS does he expect? What GS is he seeing? As TVP stated, the ASTM standard is 5 or finer (a larger number is a finer grain). This is a ridiculously easy requirement to meet; after Q&T most low-alloy steels will have a GS of 7 or 8. Each unit increase in Grain Size doubles the number of grains seen microscopically at 100X, so the difference between what is seen typically and what the sepcification allows is readily noticeable, even though both are accepatble.

To give better guidance, we need more inforation. What GS is being seen? What GS is expected? What is the heat treat condition. Will the part receive additional heat treatment?

rp

 

[stanweld]

Large grain size in forgings can be common due to excessive time the reforging billet was left in the furnace, the amount of hot work and forging temperature. A grain refining (normalizing or quench & temper) heat treatment should be employed after forging.

 

[Maui]

What you are referring to is the prior-austenitic grain size, and there are a number of different grain size tests that have been developed to quantify it. The one that is employed for a specific application depends upon the material in question and the specification requirements that are called out. For 8620H, ASTM E112 is often applicable. However, the Shepherd fracture grain size test may also be employed in certain circumstances. For high speed steels and tool steels, the Snyder-Graff grain size test is typically specified. As TVP pointed out, the condition the material was ordered in may also impact the grain size requirement.

More information about your specific problem needs to be provided in order for us to supply you with a worthwhile answer.


Maui

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