Under certain conditions it is advisable to pre-heating the parts to be welded. The most common reason is when temperable steels are welded. Welding produces a very high temperature area, higher than the quenching of materials. If the parts are cold, there is a sudden loss of temperature, by transmission, in the liquid/solid transition zone resulting in hardening by hardening with the result of a hard and fragile (warm) transition zone that can cause breakage in requests. Pre-heating decreases the temperature gradient and therefore heat transmission, so the cooling curve has a smooth slope that prevents the tempering of the transition zone.
The welding process is as follows:
- Induction heating of elementary parts, with times (differentiated for each component) controlled by programmable automaton.
- Placement of the elementarys in the useful positioner.
- The system measures the temperature of the area to be welded using a sensor. The elemental parts are heated a little more than the target temperature so that, once assembled, homogenization can occur.
- The system gives a luminous and sound warning that the target temperature has been reached in the area, allowing welding. The welding group is locked by the system until the measured temperature is within the pre-set range. There is also the possibility to set the lower limit, so the system acts as a Poka Yoke and can only be welded in the pre-set temperature range.
- The system records and archives the existing temperature data at the time of welding.
- The weldment is removed and inserted into the guard of a thermal insulation that ensures slow cooling to prevent hardening in the welded area caused by a high cooling rate.
The upper image corresponds to a micro hardness test performed on a MAG weld. Part 1 is made of HARDOX500, a CrMn steel of hardness 500 HBW. Part 2 is made of 42CrMo4, and previously tempered. In the image you can see three zones - Part 1, Part 2 and Metal contributed - leaving the transition zones sharp. The micro-hardening test is performed in accordance with UNE-EN ISO 6507-1:2018 and the hardness measured in Vickers. The starting hardness of Parts 1 and 2 is in the range of 290-320 HV. The hardness of the provided metal is in the range of 220 HV. The hardness of the transition zone is in the 240-310 HV range, with good penetration.