Casting high strength low alloy steel, A487 7-A and others 1

[friendlymonkey]

We’re looking to change to investment castings to replace components currently produced from plate that need fairly high strength and are subject to bending forces, we currently use Chinese HSLA plate with minimums of 690 MPa yield & 800 MPa UTS with about 15% elongation.

We produce varying sizes that have wall th from about 10 mm up to 30 mm, which want to be thru-hardened at least 1/3 to give good bending resistance.

In researching steels in the American standards domain we now have a short list of candidates

[ol 1]
[li]ASTM A487 grade 7-A (Cr-Mo-Ni-V)[/li]
[li]8630 as A732 grade 14Q (Cr-Mo-Ni)[/li]
[li]4130 as A732 grade 7Q (Cr-Mo)[/li]
[/ol]

All Q&T, temperatures suitable to achieve the above mechanical properties.

The steel needs to be welded to other casting(s) and is also hot dip galvanized (currently we bead blast, avoid pickling).

Foundry has already ok’ed the 8630 (14Q) and 4130 (7Q), but I’m particularly interested in the A487 7-A, because 1) its chemical composition is lower carbon than the others, and also closer to the wrought plate we’re already using, and 2) its minimum mechanical properties listed in the standard are identical to our requirement, while implying slightly better ductility than the 7Q and 14Q.

Unlike the others I can’t seem to find much about this steel despite trying to research it, and A487 doesn’t give any cross-referenced 4-digit AISI numbers unlike A732. The low carbon and vanadium doesn’t seem to match up to any that I looked at. Also, the standard annotates the 7-A grade as “Proprietary steel composition”, and I don’t understand the significance of that. The standard dates from 1963 according to its notes so perhaps it’s irrelevant now but I’d appreciate any insight?

I'm just looking for any pointers or info on this steel in particular, or any other comments are welcome.

 

[bobjustbob]

I expect you are going to have a tough time finding a foundry to pour A487 grade 7A. We would not touch anything that required boron. A little bit goes a long way and the possibility of contaminating non-boron containing grades was too big of a risk. The alloying involved is going to make developing the weld procedure interesting. I have never welded a boron containing steel but to achieve the 115ksi tensile the filler metal is pretty highly alloyed, add some boron to that at the weld interface and you could have a mess. Again, I have never worked with a boron containing steel so take that with a grain of salt.

If you came to me to suggest a 115/100ksi steel without impact, I would have suggested 8625. 8630 will do just fine.

 

[arunmrao]

8625 or 8630 are good options. Perhaps the mindset towards Boron addition comes from using HSLA steels from SSAB etc.

 

[friendlymonkey]

Thank you chaps for those comments.

On the boron issue, I notice that the max B in the HSLA Chinese Q690D plate that we're currently welding is spec'ed at 0.004% which is mid-range of the A487 7A spec (0.002–0.006). We will discuss this with the foundry and the welders, but perhaps it's a reasonable inference they are already accustomed to it.

Regarding 8630 or 8625, I would appreciate any further insight on selecting a particular ASTM standard for investment casting. At present, I'm looking at A732 (grade 14Q = 8630), mostly because that standard is a little more flexible both in chemistry ranges and mechanical properties. 8625 is not listed therein, but I suppose a carbon level at the low end of the range for 8630 (0.25-0.35) may be considered 8625? And this would suit us fine as we don't need the higher carbon.

We also looked at A958 ("Requirements Similar to Standard Wrought Grades"), which lists 8625 and 8630 both as specific grades. But, its chemistry is a little more restricting - for example silicon is 0.3-0.6%, which puts even the minimum above the ideal for HDG, and it mandates particular steps of tensile classes none of which give us exactly 100/115 ksi, so our particular specs would technically be non-standard I suppose.

Is A732 14Q a good bet or have I missed something better?

 

[bobjustbob]

The ASTM standard will depend on what type of service it is intended for. If it is in a pressure containing service like ASTM A487 then you would need a standard for pressure containing material. If it is structural you could use a standard like ASTM A148 grade. That is pretty wide open as far as chemistry goes.

Bob

 

[EdStainless]

If you are looking for consistent results then select the more restrictive specification.
Any increase in cost will be offset by higher reliability.
The more consistent the chemistry and properties are the more uniform the results of machining and welding will be.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Guest102023]

8625 or 8630 are good options.

For weldability, not so much.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"