Welding Hardox Steels
Hardox is a trade name of SSAB (equivalents are available from other manufacturers).
It is wear plate, designed to last longer than standard mild steel. There are various hardness levels with 400 and 500 being the most common.
It is relatively lean in alloy content and as such is not that prone to cracking especially if a few rules are followed.
Hardox is weldable using 7018 electrodes or standard mild steel MIG wire (SG2) but will need preheat for thick sections.
Hardox blade and teeth on digger bucket (Flynns Buckets)
Hardox 400 40mm combined thickness will need 75°C preheat whilst Hardox 500 20mm combined thickness will need 100°C. Thicker sections will need higher preheat, and in both grades the interpass temp should be kept to 150 - 175°C.
If preheating is a problem it is possible to weld without preheat using a 309L type electrode or wire, but remember stainless cannot be burned with standard oxy-acetylene. Some of the digger bucket repairers once warned me of this problem which gives them headaches on subsequent repairs.
Torch used to preheat hardox (Flynns Buckets)
Another consideration is that the weld will be softer than the plate. If this is a potential problem the weld can be capped using a hard facing electrode or wire. A single pass with a 600 hardness consumable should match or exceed the hardness of the plate. 2 passes will exceed.
These hardness numbers are Vickers, there is also Brinell and Rockwell hardness scales. Basically all are tests that measure the indentation made by a standard shaped object (either a pyramid or a ball) under a standard load.
A multiple pass weld (Flynns Buckets)