2006-12-02 08:31:46 · 2 answers · asked by absynthian 6 in Science & Mathematics ➔ Engineering
"Chromoly" refers to a class of steels containing chromium and molybdenum additions to enhance hardenability during quenching. Unfortunately, this can work against us during welding. Virtually all of the cooling of the heat-affected zone (HAZ) in the base metal around a weld is accomplished by conduction into the surrounding, cooler metal. If the rate of cooling is rapid enough, the HAZ can harden and become susceptible to something called hydrogen cracking. (Welders usually call it "cold cracking.)
Preheating the base metal reduces the temperature differential between the HAZ and surrounding metal, thereby slowing down the HAZ cooling rate. It also allows hydrogen to disperse for a longer period of time, and helps reduce restraint a little bit.
The required preheating temperature depends on several factors, including the steel's carbon and alloy content (how *much* chromium, molybdenum, etc. is present), the thickness of the base metal (this affects both cooling rate and restraint), the per-unit-length heat input of the welding process, and the expected hydrogen content of the weld metal. The latter depends mainly upon the welding process and consumables being used.
Some thin-section chromoly steels, such as the 4130 tubing often used to build race car frames, usually require no preheat at all. They're simply thin enough not to "quench" the HAZ.
2006-12-02 13:07:47 · answer #1 · answered by Anonymous · 2⤊ 0⤋
Welding is too much informations , and heat it helps makes more homogeneous surfaces of welding
2006-12-02 21:14:22 · answer #2 · answered by Eyad E 3 · 1⤊ 0⤋