welding of hot rolled and normalized steel

Hot-rolled and normalized steel in low-alloy high-strength structural steel and alloy structural steel species with a low total content of alloying elements, and the mass fraction of carbon is generally less than 0.25%. Therefore, on the whole, the weldability is better. The general strength level is relatively low hot-rolled steel, such as Q295 (09MnV), Q295 (09MnNb), Q295 (12Mn), etc. weldability is close to that of low carbon steel. However, as the strength level increases, the weldability gradually becomes worse, such as 18MnMoNb, 14MnMoV and other normalized steels, which require some process measures to be welded.

This type of steel generally does not have the problem of hot cracking as long as the content of C, S and P does not exceed the standard and does not produce local area segregation, and at the same time, the C, S and P in the weld metal is strictly controlled.

However, due to the addition of a certain amount of alloying elements in the steel, the hardening tendency of the steel increases and cold cracking problems exist. Especially for steels with higher strength levels such as 18MnMoNb, 14MnMoV, etc. the cold cracking tendency is relatively large. The cold cracking produced is generally delayed cracking, so the welding requires strict control of the hydrogen in the weld, the cooling rate of the joint and the constraint of the joint.

In this type of steel, hot rolled steel contains no or only a small amount of carbide forming elements and is not sensitive to weld reheat cracking. For the normalized steel containing strong carbide-forming elements, there is a tendency to reheat cracking. But the sensitivity to reheat cracking is related to the alloying system of the steel, for example, normalized steel Q420 (15MnVN), although it contains strong carbide forming element V, but production practice shows that it is not sensitive to reheat cracking.

When hot-rolled and normalized steel has sheet sulfide or laminated silicate inclusions along the rolling direction, or dense inclusions of alumina in the same plane, this type of steel also has the sensitivity to lamellar tearing.

Joint embrittlement is mainly produced in two parts: superheated zone and heating temperature of 200 ~ 400 ℃ aging zone.

Superheated zone embrittlement, for hot-rolled steel, mainly by the serious growth of grains, and produce superheated tissue (such as Weissite) caused by, therefore, when welding hot-rolled steel, try not to use too large welding heat input; for normalized steel, superheated zone embrittlement problem is mainly related to the precipitation phase dissolution and grain growth. Welding, if the heat input is too large, the original parent material in the normalized state of diffuse distribution of TiC, VC, VN dissolved into the austenite, so that these compounds plasmas inhibit the role of austenite grain growth is greatly weakened, so that the grain of the superheated zone grows significantly; and due to grain coarsening caused by the increase in austenite stability, the superheated zone in the cooling, but also easy to produce on the bainite, M-A group elements and other brittle large Organization, thus making the superheated area seriously embrittlement. Therefore, relatively speaking, the superheat sensitivity of normalized steel is greater than that of hot-rolled steel.