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Blog January 7, 2016

by Bob Miller

Hardfacing Preheating Part 1


Bob Miller Image
Fig. 1 Preheating can involve the whole workpiece or just a small section of it. Each will dictate the preheat temperature at which to successfully weld.
Fig. 2 Tempilsticks? are often employed to measure the preheat and interpass temperatures. Quite often two are taped together to determine a range.
Fig. 3 For those sections which require a continuous application of heat, electrical preheaters are employed. They can also be programed to ramp up, soak, and ramp down to effectively control the whole heat cycle.

Preheating in hardfacing procedures is the proper application of heat to the workpiece to achieve a number of things.

  • • Aid in accomplishing a proper cool down after welding.
  • • Rid the workpiece of any entrapped hydrogen after welding.
  • • Prevent excessive hardness in workpiece heat-affected-zone (HAZ).
  • • Avoid unwanted structures or hardness in the weld deposit.

Now that we understand the general purpose of preheating, understanding what preheat temperature to start hardfacing is a far more detailed procedure. I use a rule of thumb that will give me a pretty good estimate to start with. If I consider the type of steel I am welding on, I take its carbon content x 1000 + 100 to arrive at a Fahrenheit preheat temperature. Example: 4140 has 0.40 carbon, that would equal 0.40*1000+100=500ﾧF. I will then scale this up for very large workpieces and scale back for small workpieces. The larger the workpiece the higher the preheat temperature will be to compensate for such a large heat-sink.

So far so good, but this begs the question: how does the hardfacing rod/wire chemistry figure in? The answer is that it does become a factor and should be weighed against the base material chemistry. If I use the same formula as I did above, for the rod/wire preheat calculation and my carbon content in the wire is 0.50 carbon, then the preheat would be 0.50*1000+100=600 ﾧF. Since this preheat temperature is higher than the base chemistry preheat, I would choose the former because the 600ﾧF is the maximum preheat temperature between the two. While this may appear to be pretty straight forward, I can assure you that it can get complicated. Much depends upon the function of the workpiece and the function of the weld deposit. It may come down to a compromise between the two. When in doubt it is always advisable to contact me or the Postle office and ask for technical help.
