I would like to weld a piece of hardened steel but i don't want to ruin the treatment on the tip of the piece. Can i put some ice in the middle of the shaft so the heat generated from the welding (SMAW DCEN) doesn't temper the tip? Thanks in advance.
It will reduce the weld HAZ some , but not much. It depends on the dimensions and the heat input of the weld, the alloy , the original hardness , and the original heat treatment. The primary cooling is by conduction which is affected by the temperature difference. So the temperature difference will be 75 F / 2800 F , or 32 F /2800 F , not much of a difference. And, if the steel has much carbon ( eg. over 0.15% ) , the weld HAZ will contain some hard brittle zones with a significant chance of cracks.
Second, more planned answer . Hard steel ( Rockwell C 35 +) is not, can not be welded. In an unfortunate situation where it must be attempted ; 1 - Soften at about 1300 F , one hour . 2 - Preheat to 400 F ( I have recommended up to 600 F) . 3 - Weld. ( maintain preheat ). 4 Inspect for cracks. 5 - Soften the weld HAZ at 1300 F. 6 Heat-treat , quench and temper, according to the alloy.