What possible material conditions would result from welding non-heat treated cast 8620 that should be taken into consideration in a load bearing application?
I would rather not have any pre or post weld treatment. The cast part in question is a seat post pivot that will be used on durable medical equipment. The question from our investment casting vendor is if we need to heat treat the part prior to welding it onto the seat post. The information that I have on 8620 is that the yield strength is 80,000psi which would be sufficient for the application as long as there aren't any strange effects from the welding process. Hope the additional information helps.
Are you sure about the reported yield strength of 80 Ksi with no heat treatment for this material?? The cast version of AISI 8620 should be purchased under ASTM Specification A 958-96, which is titled "Steel Castings, Carbon and Alloy with Tensile Requirements and Chemical Requirements Similar to Wrought Grades"
The cast equivalent to wrought AISI 8620 is J12095 in A 958-96, and is specified in various heat treatment conditions - annealed, normalized, normalized and tempered and quenched and tempered.
In looking over the tensile requirements specified in ASTM A 958-96 for the cast version of AISI 8620, to meet the required 80 Ksi yield strength in your application it must be specified and purchased as Grade 105 (Ksi Ultimate Tensile Strength)/85 (Ksi Yield Strength). According to my reference material, the casting will most likely be supplied in a quenched and tempered condition. The normalized, or normalized and tempered heat treatments will not achieve the desired yield strength level of 80 Ksi because the carbon content is too low.
Welding can be done, but you will need preheat and post weld heat treatment (stress relief) to reduce the hardness and to relieve residual stresses after welding. The post weld heat treatment temperature must remain at least 50 deg F below the original tempering temperature of the casting to avoid effecting bulk tensile properties.
There is an alternative to post weld heat treatment. You can evaluate temper bead welding. I have not worked with this material specifically. It would require you to perform a weld mock-up and evaluate the temper bead method by destructive examination. It might be worth the effort considering the carbon content is only 0.20.
