(* Described as 1.7225 42CrMo4 in EN 10083 - 1: 2006; previously 709M40 in PD 970: 2001)
This procedure should be followed if joint is under high restraint. Where restraint is low, some relaxation of the following procedures may be possible.
- Preheat slowly to a temperature of 250°C either:
- In a furnace
- By resistance heating mats
- By induction heating coils
- By suitably disposed gas torches.
Check temperature by means of temperature indicating crayons or paints or by thermocouples used a short distance away from the part that is being heated. A typical distance would be 4 x component thickness.
Maintain preheat during welding.
- For MMA (SMAW) welding, use basic coated electrodes, e.g. AWS E7016 or E7018, preferably vacuum packed, or dried at the temperature and time recommended by the electrode manufacturer to give a weld metal hydrogen content below 5ml/100g. It should be noted however, that depending upon the parent material heat treatment condition, these electrodes may not match the tensile strength of the parent steel. Alternatively, use alloyed basic coated electrodes giving welds with yield strengths matching the alloy steel. MIG/MAG (GMAW) welding or flux cored arc welding can also be used with consumables giving weld metal of appropriate strength.
- When welding is completed allow components to cool to 200°C before tempering
- Heat to 620 - 650°C at a maximum heating rate of 100°C/hr or 6000/thickness (mm) °C/hr, whichever is the smaller. Temper for 2 hours. Cool to 200°C at controlled cooling rate of 100°C/hr maximum. Cool to room temperature in still air. In very highly restrained conditions, it may be necessary to consider an intermediate, lower temperature stress relief treatment.
- If it is not convenient to hold the component at an intermediate temperature after welding, pending heating for tempering, the following alternative procedures (below) could be applied. However, if restraint is very high this alternative procedure may not prevent the occurrence of hydrogen induced cracking.
- Maintain preheat at 250°C and soak for 4 hours minimum.
- Cool to room temperature at a maximum rate of 50°C/hr.
- Temper when convenient.
- Machine component to finished size
- Inspect by dye penetrant, magnetic particle or ultrasonic examination
- Repair any defects. The precise procedure necessary for repair of defects will depend on their position and extent. For example if they can be repaired without causing a hardened heat affected zone a tempering treatment will not be necessary.
- The preheating temperature in clause 1 may be reduced by 50-75°C if austenitic stainless steel or nickel based electrodes are used. Nickel based consumables have the advantage that their coefficient of thermal expansion more closely matches that of the ferritic steel, thus reducing the risk of cracking in highly restrained situations.
Suitable electrodes are: AWS E309 (23.12.L to BS EN ISO 3581: 2012); AWS E312 (29.9 to BS EN ISO 3581: 2012); AWS A5.11 ENiCrFe-3; AWS A5.11 ENiCrMo-3.
When stainless steel or nickel based electrodes are used the intermediate heat treatments recommended in clauses 3 and 5 are not necessary unless restraint is very high. Cooling to room temperature after welding can be carried out but tempering will be required if a brittle heat affected zone is unsuitable for service conditions.
It is important to note that these procedures, a) may NOT give the mechanical properties required by design considerations and b) will result in weld metal and HAZs which do not match the parent material.