Hi Allen

I have not actually welded this myself, but I can give you a reasonable idea, based on some calculations & research.

The first matter of concern is that 420 Stainless is a relatively wide term, because from my info, it only has a minimum C content. I will however take the typical C content to be 0.25%. Depending on the actual C content, some of the recommended temperatures may change.

The two main considderations here are:

1) Getting a weld that does not crack.
2) Getting a weld that can perform the service required.

Typically these alloys are used either as a type of tool steel, or for high temperature applications. If you need high strength or other properties of the 420, then you will best weld it with a matching filler. These are available. If the design of the component is such that you can get away with a lower strength weld, then it is generally easier to weld with a Ni base filler. The Ni base filler will in any case be your best bet with the 420 to 316 weld.

Welding with matching filler for 420 - 420, do the following:

1) Pre-heat to above 250 or 300°C. This will keep the weld and heat affected zone in the austenitic range, and will allow you to minimize cold cracking tendancies. (Hydrogen cracking.)
2) Maximum interpass is not so important, just keep the minimum interpass to above the pre-heat temperature.
3) Keep temperature at 300°C for 1hr per 1/2" thickness. (Rule of thumb here, not accurate.)
4) Following post heat completion, bring temperature down slowly to around 90°C. This is to ensure a full martensite transformation takes place. - Keep here for around 30min to ensure full section has reached lower temp. (If cracking occurs here, then you either have an extremely highly constrained weld, or you have too much H, or both.)
5) Then take up to post weld heat treatment (Tempering) temperature. This will depend on your final mechanical properties required. Typically 750°C for general engineering applications, 450°C for springs, 200°C for hard edge applications such as cutlery. Typically keep at tempering temperature for 1hr per inch thickness, with a min of 1 hour.
6) Slowly bring temperature down to ambient.
7) Perform hardness tests to establish if hardness is correct. If too high, perform another tempering cycle. These types of materials often require a double temper.

For welding with Ni based filler (e.g. Inconel 82), use as follows:

1) Weld buttering layer on 420. (Minimum of 5mm thick.) As long as low H consumables are used, pre-heats are not so essential. If you feel a need for pre-heat, use around 150°C. (The Mf temperature.)
2) If a pre-heat was used, bring down to below 90°C.
3) Post weld heat treat as described above.
4) Test hardness of HAZ. If too hard, perform temper cycle again.
5) Weld together (or to 316) using Ni filler.

The above procedures can only be seen as guidelines, because there are many variables that are not known.

Hope this helps.

Regards
Niekie Jooste
Fabristruct Solutions