Weld Preheating Low Alloy Steels

Low alloy steels are defined as consisting of less than 10.5% Ni, Cr, Mo, and other alloy elements. In general, low alloy steels are required to be preheated to some temperature (T_PH), prior to welding. It has been suggested that T_PH for any given steel should be about 50 F above the martensite start temperature (M_S) for the particular steel being welded. Most low alloy steels, however, have fairly high M_S temperatures, making welding at or above them somewhat uncomfortable to the welder, thereby potentially compromising weld quality. For such steels, therefore, manufacturers often opt for T_PH temperatures below M_S. A case in point is AISI 4130 with an M_S of 700 F;  For this steel, federal, military, industry and company specifications typically list T_PH temperatures in the 200-600 F range, all below M_S.

Why Preheat?

Preheating drives moisture and other contaminants off the joint; moisture, lubricants and other contaminants are sources of hydrogen. More importantly, preheating serves to reduce the rate at which the metal cools down from the welding temperature to T_PH. This is so whether preheating is above or below M_S. Cooling rate reductions will lead to a general reduction in residual stress magnitudes, and also allow more time for hydrogen removal.

Most low alloy steels that may be susceptible to hydrogen-induced cracking transform from austenite during cooling through the 800-500 C (1470-930 F) temperature range. The length of time a steel spends in this range during cooling, will establish its microstructure and, hence, its susceptibility to cold cracking. To maximize cracking resistance, a microstructure that is free of untempered martensite is desired; that is, the austenite would have transformed to ferrite + carbide and no austenite will be available to transform to martensite upon reaching M_S.

For more information about preheating low alloy steels, contact Hotfoil-EHS at 609-588-0900 or by visiting their web site at https://hotfoil-ehs.com.

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Dissimilar Metal Junctions (How Thermocouples Work)

When two dissimilar metal wires are joined together at one end, a voltage is produced at the other end that is approximately proportional to temperature. That is to say, the junction of two different metals behaves like a temperature-sensitive battery. This form of electrical temperature sensor is called a thermocouple:

This phenomenon provides us with a simple way to electrically infer temperature: simply measure the voltage produced by the junction, and you can tell the temperature of that junction. And it would be that simple, if it were not for an unavoidable consequence of electric circuits: when we connect any kind of electrical instrument to the thermocouple wires, we inevitably produce another junction of dissimilar metals. The following schematic shows this fact, where the iron-copper junction J1 is necessarily complemented by a second iron-copper junction J2 of opposing polarity:

Junction J1 is a junction of iron and copper – two dissimilar metals – which will generate a voltage related to temperature. Note that junction J2, which is necessary for the simple fact that we must somehow connect our copper-wired voltmeter to the iron wire, is also a dissimilar-metal junction which will also generate a voltage related to temperature. Further note how the polarity of junction J2 stands opposed to the polarity of junction J1 (iron = positive ; copper = negative). A third junction (J3) also exists between wires, but it is of no consequence because it is a junction of two identical metals which does not generate a temperature-dependent voltage at all.

The presence of this second voltage-generating junction (J2) helps explain why the voltmeter registers 0 volts when the entire system is at room temperature: any voltage generated by the iron-copper junctions will be equal in magnitude and opposite in polarity, resulting in a net (series-total) voltage of zero. Only when the two junctions J1 and J2 are at different temperatures will the voltmeter register any voltage at all.

Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

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