|Welding of CPM Alloys
Because of their very high alloy content, welding wear-resistant CPM grades presents a challenge to even the best welders. By following certain guidelines, however, the risk of problems related to welding may be minimized.
In general, CPM grades should be treated like high speed steels. Preheating and postheating are critical to prevent weld cracking. Even when welded ideally, the CPM grades will lose their P/M structure in the weld fusion zone, where they melt during welding. Thus, in these areas, these grades will have lower toughness than normal. For that reason, even in the best of conditions, welding on sharp cutting edges or heavy impact areas will always present some risk of cracking in service.
The following table gives brief guidelines for how to handle the CPM alloys during welding. The text gives more detailed explanations.
Preparation for welding
Whenever possible, the tool should be welded in the annealed condition. An annealed tool has the best resistance to cracking during welding. If necessary, these grades may be welded in the heat treated condition with proper precautions. In all cases, welding of these grades presents some inherent risk of cracking or weld failure.
Welded tools are not as tough as the original tools, because the welded area has a much coarser structure. The fine, uniform CPM structure is lost when the steel is melted. Precautions in service, such as careful control of alignment, fit, etc., as well as periodic stress reliefs if the tool is in a severe operation, may help prolong the service life of a repaired tool.
Most of the CPM grades must be treated like conventional high speed steels (M1, M2, T1, etc.) Grade recommendations for filler material are listed in the table.
High-speed weld rod is generally recommended for CPM 15V, CPM l0V and all the REX high speed grades, for two reasons. First, the CPM materials are normally used because of their wear resistance, and the high speed steels offer the next best wear resistance to the CPM alloys. Second, most high speed steel heat treatments (austenitizing, air cooling, tempering temperatures, etc.) are compatible with those required for the CPM grades, and therefore pre- and post-weld heating requirements will be consistent between the base and filler metals.
CPM 3V, CPM 9V, S90V (420V) and CPM S60V (440V) may require other filler material. See the table and text, or consult your Crucible representative, for more information.
All of these grades must be preheated prior to welding, and maintained at a high temperature during welding, regardless of the condition of the tool. See the table for specific temperature recommendations.
When welded in the annealed condition, all grades should be preheated to 800 to 1200°F (430 to 650°C), and maintained above 800°F (430°C) during welding.
When welded in the heat treated condition, the preheat temperature must be more closely controlled to prevent altering the original heat treatment. Most grades should be preheated to 800 to 1000°F (427 to 538°C). Heat treated CPM S90V and S60V, because of their lower tempering temperature range, should be preheated to just below the original tempering temperature to avoid loss of hardness.
Preheating serves several functions. First, the tool is tougher at higher temperature, and therefore more resistant to cracking for any reason during welding. Second, these steels are all air-hardening, and therefore areas heated up during welding will transform to the as-quenched (untempered) structure on cooling. This change in their microstructure occurs on cooling from the welding temperatures at or below about 600°F (315°C). Keeping the steel above about 700°F (370°C) prevents the welded area from hardening while the welding is going on. Thus, the transformation will eventually occur more evenly throughout the weld area. Finally, a preheated tool is subject to less thermal shock than a cold tool, and there is less stress due to adjacent hot and cold areas.
After the welding is completed, let the workpiece cool down uniformly to hand warm, or 125°F (52°C).
Once again, these grades are all air-hardening. Therefore, because of the high temperatures generated during welding, there will always be some areas in the as-quenched condition. For this reason, the tool should be post-heated, or tempered, immediately upon reaching hand warm (125°F, or 52°C).
If the tool has been welded in the annealed condition, it may be tempered for 6 to 8 hours at 1350 to 1425°F (730 to 775°C). Ideally, it should then be reannealed before any subsequent heat treatments. This can be done by heating it to 1600 to 1650°F (871 to 899°C), holding for 2 hours, cooling to 1400°F (760°C), holding 6 to 8 hours, and then air cooling to room temperature.
If the tool has been welded in the heat treated condition, then tempering is required for the heat-affected areas. Most CPM grades should be tempered at about 1000 to 1025°F (538 to 552°C). Some grades may require different temperatures due to other considerations; see table and text below. All grades should be tempered at least twice, and triple tempering is usually recommended. Each temper should consist of holding for two hours at temperature, and cooling all the way down to room temperature between tempers. If there is any doubt about the proper temperature to use, simply use 25 to 50°F (14 to 23°C) below the tempering temperature used in the original heat treatment.
Certain CPM grades require slightly different welding procedures than those shown above.
CPM S90V (420V) and S60V (440V) should be welded using filler metal compatible with D-2 or 440 C stainless, rather than high speed steel. Preheating and postheating of heat-treated material should be performed at a temperature just below the original tempering temperature. The usual tempering range for this grade is 400 to 750°F (205 to 400°C). Preheat and postheat temperatures for annealed material are the same as other CPM grades.
CPM 3V may be welded using air-hardening tool steel filler material, such as A2 type. Preheat tools to 750/900°F (400/480°C), and post-heat at 950/1000°F (510/540°C). This post-heat may result in hardness in the weld deposit of about HRC 54-56, but is necessary to ensure adequate stress relieving and toughness.
CPM 9V is used for both cold work (metal forming) applications, and hot work (e.g., forging). When used at high hardness (>HRC 50) for cold forming applications, it should be welded with low alloy high speed steel (M1 or M2 type). Preheat to 800/1000°F (430/540°C). Temper after welding at 1050/1100°F (565/595°C).
When used for hot forming applications, at hardness below about HRC 50-52, CPM 9V may be welded with a hot-work tool steel filler metal (such as used for H-13) if desired, to maintain better toughness. Post-weld tempering should be performed at about 1000°F (540°C), to maintain relatively high hardness in the weld metal. In addition, any hardening treatments performed after the welding should be done at no higher than 1950°F (1065°C) to prevent overheating the hot-work tool steel weld metal. Maraging filler material, if desired, should be used with heat treated material only, as austenitizing temperatures for CPM 9V are not compatible with recommended heat treatments for maraging steels. See table for appropriate temperatures.
(l) Low alloy high speed steel filler recommended, such as M1 or M2 type.