Dissimilar Metal Welding III: For Better Welded Steels

By September 4, 2015 Uncategorized No Comments

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Steel: A Fundamental Material

Steel is the basic industrial material. Its use indicates the state of wellbeing in an economy as well as its growth. Greater per capita steel consumption implies that more steel is used to make products that improve the quality of life and infrastructure in the economy.

Austenitic steels are the most weldable. Steel is among the most commonly used and among the most welded material in the world. This merits a special treatment.

The following is a crisp compilation of recommendations made in this regard in a paper by B. Mvola, P.Kah, and J. Martikainen of the Lappeenranta University, Finland and by the Swedish industrial company ESAB.

Considerations when Welding Diverse Steels

Heat input, type of filler material, and pre and post heat treatments govern the mechanical properties of the weld made by arc welding between steels with substantially distinct mechanical properties.

Steel in Construction  (Image Courtesy of Kuczin at BigStock.com)

Steel in Construction
(Image Courtesy of Kuczin at BigStock.com)

General observations for welding disparate steels:

  • Harder steels are less weld-able
  • Carbon Equivalent (CE) is an important property

Steels with higher CE decide the welding parameters and preheating temperatures while lower CE steels govern the choice of consumables (electrodes)

  • A consumable with very high CE promotes cracking. Stress annealing brings down this risk and so does controlled or slow cooling
  • Dissimilar metal welding (DMW) with stainless metal fillers is far tougher than with nickel-based fillers

Heat input, solidification patterns, filler material, and consumables control the efficacy of the welding process that joins disparate stainless steels (SS).

Limit the heat input and prevent the formation of the crack-prone martensite phase. Use stabilized consumables or low-carbon grade consumables when welding non-stabilized steels to stabilized ones.

Often, welding engineers select filler compatible with multiple stainless steels. Choose austenitic filler metals for better mechanical properties of the weld.

Applications demand the use of steels other than SS when strength, creep-resistance, toughness, and cost are more important than corrosion resistance. Then, it is necessary to weld SS to non stainless steels.

Steel in the Automobile Industry  (Image Courtesy of bibiphoto at BigStock.com)

Steel in the Automobile Industry
(Image Courtesy of bibiphoto at BigStock.com)

Current, welding speed, and filler material influence the elongation and ultimate tensile strength of a weld between stainless steel and low-alloy steel / carbon steel. Choose welding parameters diligently to better control the composition of the weld zone.

Hydrogen cracks and solidification cracks are the main forms of fracture that crop up while welding stainless steels to low-alloyed / unalloyed steels.

Unclean groove surfaces, moisture-infested electrodes, and adverse external conditions induce hydrogen cracks. Metallic impurities from the unalloyed or low-alloyed base material cause solidification cracks. When making such welds:

  • You do not normally require preheating
  • Use stainless consumables that are more heavily alloyed than SS
  • Fill the groove completely with a nickel-based or an over-alloyed consumable. You can also butter the unalloyed or low-alloyed steel surface with the over-alloyed stainless weld material and fill the groove with a consumable compatible with the stainless base metal
  • Nickel-based consumables prevent carbon migration in welded structures of different steels designed for high temperature (over 2000C) applications
  • Use nickel-based or over-alloyed consumables to minimize hydrogen cracking

The welding operation continuously dilutes consumables. Unalloyed, low-alloyed, or insufficiently alloyed consumables form hydrogen crack prone martensitic weld metal. Nickel-based or over-alloyed consumables counter such dilution

  • High manganese content consumables counter solidification cracks that are more likely to affect austenitic weld material

Marginal heating of the non-stainless metal to about 1500C is useful but not necessary when welding mild or low-alloyed steel to stainless steel. Such heating discharges hydrogen from the martensitic weld metal and lowers the hazard of hydrogen cracking.

Steel in Power Plants  (Image Courtesy of Christian Lagerek at ShutterStock.com)

Steel in Power Plants
(Image Courtesy of Christian Lagerek at ShutterStock.com)

Low-alloyed consumable of a composition compatible with base metals gives good weld joints between mild steel and low-alloyed steel. Mild steel determines the choice of the consumable while low-alloyed steel influences process parameters.

You improve the weld’s bearing quality and rupture energy by welding mild steel (MS) to carbon steel. It is however difficult to weld MS to carbon steels. Tensile strength increases and hardness decreases when you lower welding heat input.

Choose a consumable with a CE around the value of the CE of the less hardenable steel when welding disparate low-alloyed steels. Restrict the heat input to minimize HAZ width. You get reasonable weld joints for steels with different CE provided both steels are of the same main type.

Welding heat input is the primary factor governing the quality of welds between structural steels, carbon steels, or bearing quality steels. Process parameters and filler metals play a secondary role.

Steel in Railways (Image Courtesy of titkosjanos at ShutterStock.com)

Steel in Railways
(Image Courtesy of titkosjanos at ShutterStock.com)

Mechanical and chemical properties of the weld improve with proper consumables and heat input control when welding high strength low alloy (HSLA) steels to low-carbon steel (LCS) and when welding LCS to LCS.

Alloy levels and carbon content of the fusion zone (FZ) determines the FZ’s hardness and microstructure when welding HSLA to LCS. The risk of carbon migration shoots up when you don’t use fillers. Larger CE values of the FZ give harder microstructures.

HSLA possess great strength and toughness while being low on weight. They are easily weldable and formable but are extremely sensitive to welding diffusion and heat input.

Carefully set welding parameters and heat input when welding diverse HSLAs. You get least residual stresses in the weld when welding HSLAs at high speed in the opposite direction.

Excessive heat input softens the HAZ of welds in high strength steels (HSS). Post weld heat treatment (PWHT) takes care of this issue efficaciously.

Appropriate choice of filler metal, welding parameters, and PWHT determine the quality of weld between creep resistant steels designed for high temperature applications. Carbon migration increases when you do not use fillers. Nickel-based fillers give best welds.

When welding cast iron to steel, use nickel-based filler material only. Cast iron has limited weldability and you cannot weld certain cast irons at all. You don’t need to preheat cast iron but do remove shrinkage stresses by peening as cast iron has low thermal expansion and ductility.

Buttering is a better option than directly joining cast iron to steel when welds need to be very strong and ductile. You cannot use welding methods that form large weld pools and require high heat input.

Finally

Although the list of factors that control weld quality is not very large, the relative importance of each factors changes for different pairs of steels. Furthermore, it is the interplay between these factors that govern the final quality.

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