Preheat on Hardened Steel: What the Mill Cert Tells You and What Happens When You Skip It

I work a lot with hardened and tempered steels. Pins, shafts, structural components on forestry conveyors and heavy mining equipment. These parts come in quenched and tempered, and they need to go back into service with their mechanical properties intact. That means the welding procedure has to be right, and it starts well before you strike an arc.

The most important document on a job like this is the mill cert. Not the material designation on the drawing, not what the supplier told you over the phone, but the actual cert for that heat of steel. I pull it every time, without exception. The reason is simple. A designation like 4041 or 4140 is a family, not a fixed chemistry. Two bars with the same name from different heats can have different carbon, manganese, and chromium content, and those differences move the carbon equivalent in ways that matter.

The CE(IIW) formula is what I use: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. You plug in the weight percentages directly from the cert. For a typical 4041 chrome-moly shaft, you’ll land somewhere between 0.55 and 0.70. Anything above 0.45 means preheat is mandatory. At 0.60 and above, you’re dealing with a steel that will crack if you don’t take the procedure seriously.

What 400°F actually means in practice

On pins for heavy equipment, the target I work to is 400°F, which is around 204°C, and that temperature has to be reached and held across the full preheat zone. A minimum of three inches on each side of the weld, six inches total. I’ve seen guys heat a small area right at the joint and call it good. That’s not preheat. That’s a hot surface on a cold mass, and the thermal gradient you create that way can crack the base metal away from the weld before you even finish the first pass.

The torch work matters too. You move the rosebud continuously, covering the whole zone, letting the heat soak in rather than concentrating it on one spot. On a heavy pin, this takes longer than most people expect. Rushing it gets you a hot skin and a cold core, which is about the worst situation you can have going into a weld on chrome-moly.

For verification, I use heat crayons. You mark the steel at 75mm from the weld centerline, and when that mark goes wet and shiny, you’re at temperature. It’s direct, fast, and it doesn’t lie. I have much less use for infrared thermometers on this kind of work. The readings depend on the emissivity of the surface, and mill-scale steel is inconsistent enough that you can easily be 30 or 40 degrees off without knowing it. The angle of the gun matters too. I’ve seen people get confident readings on an IR gun and then wonder why they had cracking. The heat crayon is old technology, but it’s honest.

After the last pass

Once the weld is done, the cool-down is just as important as the heat-up. What you’re trying to avoid is rapid cooling that traps hydrogen in the hardened heat-affected zone and builds up enough stress to crack. My method in the shop is straightforward. I bury the pins in a large closed bin packed with dry sand and gravel. The thermal mass of the sand slows the cooling rate dramatically, and when I come in the next morning, the parts are still warm to the touch. That’s what a proper slow cool looks like. It’s low-tech, it costs nothing, and it works consistently.

When someone grabs the wrong wire

I was working on a hardfacing job, rechargement on a large plate. I had specified the wire: Lincoln Electric Metalshield MC-110, classified E110C-K4 H4, rated at 110 ksi tensile strength (760 MPa). It’s a low-alloy metal-cored wire with good low-temperature impact properties and a low hydrogen classification, which is exactly what you need on high-strength base metal.

A helper who was setting up while I was on another part of the job grabbed a different wire off the shelf. A standard structural wire, something in the 70 ksi range, the kind you’d use on mild steel. Not classified for this application, and he started welding without verifying preheat either.

The deposit was softer and weaker than specified. The mismatch between the deposit and the hardened base metal created stress concentrations at the fusion line, and the missing preheat meant the HAZ cooled too fast. When I came back and looked at the plate under a good light, the cracks were there. Fine and tight, the kind you can miss in bad lighting, but clearly visible once you slow down and look properly. The plate had also come in wrong by about 3/16 of an inch, so it already needed rework. The cracking made it significantly worse and delayed the delivery.

Both problems came from the same place: assumptions made without checking. The mill cert was there. The WPS was there. The right wire was in the shop. None of it was consulted before starting. That’s where most welding failures actually begin, not in the arc itself but in the few minutes before it.

Chrome-moly steels are not forgiving of improvisation. They reward the person who does the calculation, pulls the cert, and checks the temperature before every pass. That’s not excessive caution. It’s just how the metallurgy works, and the cracks will always tell you when you ignored it.