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UNDER THE 52 MoldMaking Technology —— AUGUST 2016 Image courtesy of Toolweld Inc. By Eric Hild Cracking in tool welding is a common challenge, but it is one for which risk can be minimized. It all depends on the weld location, steel type and weld- ing process. In simplest terms, crack- ing occurs due to the inability of the steel to handle the stresses of tempera- ture fluctuations caused by the weld- ing. Essentially, the steel will retract as it cools after welding. Oftentimes, cracks immediately become appar- ent, but sometimes they occur hours later. Essentially, welding improperly prepared steel always carries the risk of potential cracking. Cracking can take many forms. For example, it can occur in the base steel next to the weld or in the weld itself. In order to address cracking issues, it is necessary to determine the origin of a crack. For example, did the crack hap- pen while the piece was being welded (referred to as hot cracking), or did the crack occur hours after the piece was welded (referred to as cold cracking)? Hot and cold cracks. In hot crack- ing, the crack occurs along the length of the weld and appears as the weld- ing is taking place. Most often, this is the result of welding a hard material. Cracking also can happen if the welder is welding too fast and using more filler material than necessary. Cold cracking happens as the mate- rial cools after welding and usually occurs in the weld's heat-affected zone (HAZ), which is incapable of respond- ing to temperature stresses. Cold cracking also can happen when a large buildup of material is required (1/8 inch or more over a large area). As the repair cools, the weld's HAZ contracts (shrinks), causing a crack. The crack may even reach a point hours later where the weld will break off from the base steel. Microscopic laser-weld cracks. Laser welding steels with a high Rockwell hardness (D-2 or M-4) can cause microscopic surface cracking on the weld. Using a filler rod that is harder than the base material can easily cause this to occur. To reduce this problem, the operator should grind down a few Cracking Demystified Peening, which helps reduce the effects of steel contraction during cooling, is accomplished by beating with a punch and hammer directly on the weld. thousandths in the repair area, weld with a filler rod that is softer than the base material, then top off the repair with a hard filler that matches the hardness of the parent material. Cracks before welding. Cracks can also appear while weld- ing. Sometimes an existing crack is made worse during weld- ing, and that is what enables it to be detected. For example, molds that have crashed in service can have unseen cracks that become more pronounced when they are pre-heated before welding. During TIG welding, a crack can become noticable when the heat of the weld puddle gets close enough to open it. This is why every repair should begin with an inspection