August 2022 Volume 4

MATERIALS

per inch of thickness at temperature. A thermal blanket to partially cover the die during the welding process will retard heat loss, and offer safety protection for the welder. Attempting to preheat even a moderately sized die with a simple flame is ineffective, and possibly damaging if an overly aggressive flame is locally focused. A lingering flame in one area could induce some tempering (softening) of that area, and lead to deformation or cracking under forging loads. The suggested minimum temperature to be maintained during welding is not a hard floor. The exact martensitic-start temperature can vary a little depending upon exact chemistry, and the amount of martensite produced is proportional to the temperature drop below the start temperature. The selection of a weld metal is not usually as simple as matching the base chemistry. The weld metal may vary considerably depending upon the final properties desired for the deposit. This is crucial information that should be discussed with the welding source. The greatest risk of dropping below the suggestedminimumwelding temperature occurs upon completion of the welding operation and transfer to the furnace for post-weld, stress relief. The temperature suggested for post-weld treatment is usually about 50 ° F below the tempering temperature employed by the die steel supplier to establish the base hardness of the die block. At this temperature, the base metal will not undergo any further tempering, i.e., hardness loss, but is sufficient to promote the completion of the austenite-to ferrite transformation to a bainitic microstructure (a lower stress transformation product than martensite) and effect an adequate degree of tempering of the entire weld region, including the HAZ. The holding time for this process should be adequate for equalizing the temperature in the die block, usually about 30 minutes per inch of thickness. A slightly higher stress relieving temperature may be used to effect marginally better stress reduction, but at the risk of losing some base hardness in the die. In some cases, this may be a desirable trade-off. The exact hardness loss depends upon particular conditions, but is generally about one Rockwell C point for each 10 ° F above the base tempering temperature. The foregoing thermal treatment of weldments applies to any type of welding method. Welding methods such as conventional “stick” welding with coated rods, MIG (wire feed), TIG (uncoated stick feed into an arc between a Tungsten tip and the base metal), or friction welding all have an effect on the base metal in a small layer

surrounding the weld deposit, the HAZ. The extent of the effect can vary, however, between these methods, and this important aspect of welding die steel should be thoroughly reviewed with your welding source. Another adverse effect occurring in the HAZ is grain coarsening. Whenever the austenitizing temperature exceeds about 2000 ° F, and up to the liquidus of about 2750 ° F, the pinning effect of aluminum nitride particles that limit grain growth is overpowered, and a rapid increase in grain size results. Most steel is melted to fine grain practice to improve ductility and fracture toughness. Losing this benefit in the HAZ further contributes to cracking sensitivity. Re-heat-treating the entire die with an austenitizing temperature limited to about 1700 ° F will refine the coarsened grain in the HAZ, but this approach is impractical for a die with even a simple impression. There is a procedure that can exert a grain refining affect, apart from thermal cycling. It is a localized mechanical hot working of the welded region called peening . This technique, as with most welding processes, requires an understanding of the blow force and pattern required to be effective. Peening can be effective, but the practice should be discussed with an experienced practitioner of this technique. Traditionally, the Finkl company has strongly advised against welding die blocks, and still does. The innumerable variables involved in producing a safe and effective weld can probably only be adequately assessed by a degreed welding engineer. Unfortunately, some welding operations are undertaken on complex die alloys with limited appreciation of the difficulties posed relative to welding mild steel with low carbon filler metal. The purpose of this article is to alert anyone considering the welding of die steel, or for that matter any higher strength alloy, to the important effect upon the base metal surrounding the weld deposit and fusion zone. This region suffers a metallurgical transformation that, quite apart from all the other variables involved, can lead to underbead cracking and weld failure. Appreciating and addressing this important aspect of welding die steel with your welding source is essential to success. In 1709, the English poet Alexander Pope wrote, “A little learning is a dangerous thing; drink deep, or taste not the Pierian spring: there shallow draughts intoxicate the brain, and drinking largely sobers us again.” Alex probably had welding of die steel in mind when he wrote this. Nick Cerwin is the retired Director of Technical Services at Finkl Steel.

Die Surface

FIA MAGAZINE | AUGUST 2022 33