TIG welding is a versatile and precise process that can produce high-quality welds on various metals, including low alloy and carbon steels. However, TIG welding these steels requires some special considerations and precautions to avoid common problems such as cracking, distortion, and loss of mechanical properties. In this blog post, we will explain the basics of TIG welding low alloy and carbon steels, and provide some tips and best practices to achieve successful results.
Low alloy and carbon steels are types of steels that have a low percentage of alloying elements (such as chromium, molybdenum, nickel, etc.) and a low to medium percentage of carbon (usually less than 0.25%). These steels are widely used in various industries and applications, such as construction, automotive, aerospace, and pipelines, due to their low cost, high strength, and good weldability.
However, not all low alloy and carbon steels are equally weldable. Some steels have a higher carbon equivalent, which is a measure of the combined effect of different elements on the hardenability and crack susceptibility of the steel. The higher the carbon equivalent, the more likely the steel is to harden and crack when welded. A common formula to calculate the carbon equivalent is:
NOTE: where C, Mn, Si, Cr, Mo, V, Ni, and Cu are the percentages of carbon, manganese, silicon, chromium, molybdenum, vanadium, nickel, and copper in the steel, respectively.
For example, a low alloy steel with a carbon content of 0.30% and a manganese content of 1.20% has a carbon equivalent of 0.60%, which is relatively high and indicates limited weldability. On the other hand, carbon steel with a carbon content of 0.10% and a manganese content of 0.40% has a carbon equivalent of 0.16%, which is low and indicates good weldability.
TIG welding low alloy and carbon steels can pose some challenges, depending on the type and condition of the steel, the design and fit-up of the joint, and the welding parameters and technique. Some of the common challenges are:
Cracking: This is the most serious and common problem that can occur when welding low alloy and carbon steels, especially those with high carbon equivalent. Cracking can occur in the weld metal, the heat-affected zone (HAZ), or the base metal, and can be either hot cracking (during or shortly after welding) or cold cracking (hours or days after welding). Cracking is caused by several factors, such as hydrogen embrittlement, residual stresses, martensitic transformation, and lack of ductility. Cracking can be prevented or minimized by using proper preheat, interpass, and post-weld heat treatments, selecting suitable filler metals, controlling the hydrogen content, and avoiding excessive heat input and restraint.
Distortion: This is the deformation or warping of the welded joint due to the uneven expansion and contraction of the metal during the heating and cooling cycles of welding. Distortion can affect the dimensional accuracy, alignment, and appearance of the weldment, and can also induce residual stresses that can lead to cracking. Distortion can be reduced by using proper joint design and fit-up, clamping and fixturing, welding sequence and technique, and heat input control.
Loss of mechanical properties: This is the degradation or alteration of the strength, hardness, toughness, and corrosion resistance of the welded joint due to the thermal cycles and metallurgical changes induced by welding. Loss of mechanical properties can affect the performance and service life of the weldment, and can also increase the risk of cracking. Loss of mechanical properties can be avoided or restored by using appropriate filler metals, heat treatments, and post-weld treatments.
To achieve successful and high-quality welds on low alloy and carbon steels, the following best practices should be followed:
The filler metal should match or undermatch the base metal in terms of chemical composition, mechanical properties, and weldability. Undermatching means using a filler metal that is one or two grades lower than the base metal, which can improve the ductility and crack resistance of the weld. For example, 4130 chromoly tubing for aircraft is often welded with plain ER70S-2 mild steel TIG rod 1. The filler metal should also have a low hydrogen content, preferably less than 5 ml/100 g, to prevent hydrogen embrittlement and cracking. The filler metal should be stored in a dry and clean place, and preheated if necessary, to avoid moisture and contamination.
Preheating is the process of heating the base metal to a certain temperature before welding, to slow down the cooling rate and reduce the thermal gradient. Preheating can prevent or reduce cracking, distortion, and loss of mechanical properties, especially for steels with high carbon equivalent or thickness. The preheat temperature depends on the type and condition of the steel, the joint design and fit-up, and the welding parameters, but it is usually between 100°C and 300°C. Preheating should be done uniformly and gradually, using a suitable heat source, such as a torch, an oven, or an induction heater. The preheat temperature should be maintained throughout the welding process and checked with a thermometer or a temperature indicator.
The shielding gas protects the weld pool and the electrode from atmospheric contamination, stabilizes the arc, and influences the weld characteristics. For TIG welding low alloy and carbon steels, you can use pure argon or a mixture of argon and helium, which provide good arc stability, penetration, and cleaning action. You should avoid using carbon dioxide or oxygen, which can cause oxidation, porosity, and embrittlement of the weld metal. You should also use a proper gas flow rate, which depends on the nozzle size, the joint configuration, and the ambient conditions, and can range from 10 to 20 liters per minute. You should check the gas flow rate with a flow meter and avoid excessive or insufficient gas flow, which can cause turbulence, gas wastage, or contamination.
The welding technique involves the manipulation of the electrode, the torch angle, the arc length, and the travel speed. For TIG welding low alloy and carbon steels, you can use a straight or a slight weave motion of the electrode, which can provide good fusion, penetration, and bead appearance. You should avoid using a circular or zigzag motion, which can cause excessive heat input, distortion, and porosity. You should also maintain a constant torch angle of 15° to 20° from the vertical, a short arc length of 1 to 2 mm, and a steady travel speed of 2 to 4 mm/s. You should avoid changing the torch angle, the arc length, or the travel speed abruptly, which can cause inconsistency, undercut, or lack of fusion.
If you want to weld aluminum, you can read How to TIG Weld Aluminum: A Beginner's Guide.
TIG welding low alloy and carbon steels is a challenging but rewarding process that can produce high-quality welds with proper preparation, execution, and inspection. By following the best practices discussed in this blog post, you can avoid common problems such as cracking, distortion, and loss of mechanical properties, and achieve successful results. Remember to choose the right filler metal, preheat the base metal, control the heat input, use a suitable shielding gas, and use the correct welding technique. You can also consult the relevant welding standards, codes, and specifications for more guidance and recommendations.