Welding of problem steel

teels containing carbon in excess of 0,25%, chromium and molybdenum over 1,5% and manganese over 1,5% exhibit increased strength and hardenability and decreased weldability.

Additional elements such as vanadium, silicon, nickel, boron, niobium and titanium also influence hardenability and weldability. Steels of increased hardenability tend to form brittle microstructures in the heat affected zone, which may result in cracking. Steels featuring reduced weldability are commonly referred to as ‘problem steels’ as a result of the problem areas that are directly caused by shrinkage stresses, rapid cooling rates and the presence of hydrogen.

Electrodes for welding problem steels are chromium nickel austenitic types containing delta ferrite in the range of 10–80%. The weld metal is insensitive to hot cracking above 1 200°C. At ambient temperatures, the weld metal is strong and tough and is capable of withstanding heavy impact and shock loading in service.

Problem steels fall into two categories, i.e. ferritic types which require preheat and austenitic steels such as 11–14% manganese steels, which require minimum heat input.

When hardenable ferritic steel types are to be welded, reference should be made to the section on mild and medium tensile steels for the calculation of the carbon equivalent and preheat temperatures.

Problem steel electrodes are suitable for welding combinations of dissimilar steels such as chromium, molybdenum, creep resistant steels and stainless steels to mild and low alloy steels. Care should be taken when welding such combinations to ensure that excessive dilution between the base and weld metal does not occur.

 The Welding of Dissimilar Steels

When welding dissimilar steels, a number of factors must be taken into account. For example:

The weld metal must be capable of accepting dilution from both dissimilar base materials without forming crack-sensitive microstructures. These structures must remain stable at the desired operating temperatures.

The mechanical properties of the weld metal should be superior to the weaker of the two base materials.

The coefficients of expansion should preferably be between those of the base materials in order to reduce possible stress concentrations.

The corrosion resistance of the weld metal should be superior to at least one of the base materials to avoid preferential attack of the weld metal.

In many instances, it is not possible to satisfy all of the foregoing points and a compromise has to be made. Afrox 309 and 312 problem steel electrodes have been specially designed to weld a large number of dissimilar materials such as stainless steels to carbon manganese steels and low alloy steels, and low alloy steels to 11–14% manganese steels, high carbon and tool steels, etc.

Calculation of Final Weld Metal Structures

The final weld metal chemistry, and therefore properties, depend on the amount of dilution that occurs during welding.

Weld metal dilution is normally expressed as a percentage of the final weld metal composition, the effect depending on a number of factors such as the joint configuration, the welding technique and the welding process. With the manual metal arc process, dilution in the vicinity of ±25% can occur. This will obviously be greatest in the root pass and least in fill-in passes where two or more runs per layer are used.

The Schaeffler diagram is a useful tool, in that it allows us to determine, theoretically, the microstructures after dilution. This is illustrated by means of the following example:

Suppose we want to weld 410 steel (13 Cr; 0,8 Mn; 0,5 Si and 0,08 C) with Afrox 309Mo (23 Cr; 12 Ni; 1,0 Mn; 0,5 Si and 0,03 C), and we assume 30% dilution (the base metal contributes 30% of the union and the electrode the other 70%).

hat is the composition of the resultant weld metal?

The 410 plate is represented by point B (Cr equivalent 13,75%; Ni equivalent 2,8%) and the Afrox 309 MoL electrode by point A (Cr equivalent 23,75%; Ni equivalent 14,5%). Any resultant weld metal from this mixture of A and B will be on the line that joins them. As we have assumed 30% dilution, point C will give the resultant microstructure (i.e. austenite with 10% ferrite). This weld is therefore possible without any danger of hot cracking.

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