August 2022 Volume 4

MATERIALS

Welding can be deceptively simple: place a welding rod into the clamp, turn on the power, and tap the rod to strike an arc. Surprisingly, this minimal approach, possibly taken from a “Quick Start” pamphlet, can sometimes produce a successful weld. Why, then, do some universities offer a four-year curriculum on weld engineering? The answer is that beyond the most simplistic conditions, successful welding requires a strong measure of metallurgical knowledge of both filler and base metals. This article focuses on the metallurgical affects occurring in the specific base metal of die steel. Types of welding rod and the many other variables involved in welding high strength die steel is best addressed by deferring to accredited welding experts. Plain low-carbon steel, (such as AISI/SAE 1018) does not harden under any type of thermal cycling. The thermal cycle experienced during welding leaves the locally affected base metal in a relatively soft, highly ductile condition, around 50,000 psi/30,000 psi tensile and yield strength, respectively. It is often referred to as field weldable because no particular thermal conditions are required before, during and after twelding to avoid a hardened, brittle weld. This is the type of steel, together with a mild steel welding rod, used in basic welding courses. The success of the weld in this case is more dependent upon technique than managing complex metallurgical responses from an alloyed base metal. Die steel is, of course, vastly different than plain low-carbon steel, and even more different than mildly alloyed High Strength Low Alloy (HSLA) steel. One of the purposes of a die steel chemistry is to provide maximum hardening potential and deep response from a prescribed heat-treating cycle. Rapid cooling (water quenching) from a red-hot temperature (austenitized) produces a tensile and yield strength of about 300,000/250,000 psi, respectively, in the as quenched condition. At this high strength, the ductility and fracture toughness are correspondingly very low. The as-quenched steel must be tempered to lower the tensile and yield strength to about 200,000/180,000 psi to raise the ductility and fracture toughness to a level compatible with die steel service. Quenched and tempered die steel has an appropriate balance of strength, ductility and toughness to successfully endure the forces encountered in forging. The concern with welding die steel is that the localized area around the weld is heated to temperatures and cooled at rates that parallel the critical values used in quench hardening a die block. As with an entire die block, if this hardened area surrounding the weld remains untempered, the affected area is highly stressed and susceptible to fracture. The general schematic in Figure 1 shows the three zones of a weld: the actual weld metal deposit, the fusion zone, and the Heat Affected Zone (HAZ). Die Steel Welding By Nick Cerwin

The weld metal is selected by the welder to provide the properties required to meet the purpose of the weld, such as tacking pieces together, laying a hardened layer for wear resistance, or rebuilding an area of the die. The weld thickness, therefore, ranges from thin to very thick. The fusion zone and HAZ are always relatively thin layers, but both are critical to the integrity of the weld. Having examined a fair number of weld failures in die steel, it has been our experience that all the investigated failures were attributed to a brittle, untempered HAZ. It is essential, therefore, that all die steel welds be treated with post-weld stress relieving operation. It is crucial to success. Although the HAZ is not water-quenched in the manner of hardening a die block, the cooling rate achieved by the conductivity of metal into the massive heat-sink of the die body is sufficient to produce a brittle martensitic microstructure. This fact emphasizes the distinctly different metallurgical response of die steel compared to field weldable mild steel. Proper metallurgical treatment of the entire weld region, not just welding technique or weld metal selection, is crucial to successfully welding die steel. Providing proper thermal treatment in preparation for, during, and after welding a die block can be challenging. Many dies are large enough that there is no furnace capacity on site to accommodate the die. It is sometimes the situation, too, that time is of the essence, and getting the die back in service overpowers recommended metallurgical practice. Such limitations and urgencies are often responsible for poor welding outcomes. Recommended temperatures to be used as part of a proper metallurgical handling of die block welding are for pre-heating to establish and maintain a minimum die temperature during welding, and the post-weld stress relieving process. The purpose of preheating is tomaintain a temperature during welding that is above the martensitic-start temperature. This assures that little or no high stress, martensitic microstructure develops in the HAZ. While a tempered martensitic microstructure is desirable for the overall die block, the tempering in such a case must be performed with a critically timed tempering treatment to avoid quench cracks. Welding operations can vary in duration, depending upon the project, thereby introducing timing issues that could allow some martensitic transformation in the HAZ, thereby raising the risk of HAZ cracks. A contributory benefit of preheating is to decrease the contraction differential between the weld deposit and the surrounding base metal to mitigate cooling stresses. A normal temperature range for weld preheating of most widely used die steel grades is 400 ° -800 ° F. Considering that most die blocks are of considerable mass, it is essential to have an appropriate furnace available to provide an adequate thermal soak, perhaps 30 minutes

FIA MAGAZINE | AUGUST 2022 32