In our previous discussion, we began our discussion on How to Weld High Carbon Steels. We explored the fundamental characteristics of low, medium, and high carbon steels, highlighting the unique challenges high carbon content introduces to the welding process. We established that the increased hardness and brittleness of high carbon steel make it highly susceptible to cracking, particularly hot and cold cracking, and necessitate careful management of the Heat Affected Zone (HAZ). Now, we will build upon those foundational concepts, delving into the specific welding processes best suited for high carbon steel,  filler metal selection, and optimized welding techniques that are crucial for achieving robust and reliable welded connections.

 

Welding Processes for High Carbon Steel

While many welding processes exist, certain ones are more commonly employed or recommended for welding high carbon steel due to their ability to control heat input, manage hydrogen, and provide adequate shielding.

 

Shielded Metal Arc Welding (SMAW) / Stick Welding

Shielded Metal Arc Welding (SMAW), often referred to as stick welding, is a versatile and widely used manual process that employs a consumable electrode coated with flux. This flux coating disintegrates during welding, producing shielding gases that protect the molten weld pool from atmospheric contamination and slag that covers the cooling weld. SMAW is a common choice for carbon steel welding and is particularly effective for thicker sections of high carbon steel.

A significant advantage of SMAW for high carbon steel is the availability of low-hydrogen electrodes (e.g., EXX18, EXX15, EXX16 classifications). These electrodes are crucial for minimizing diffusible hydrogen in the weld metal, which is a primary cause of hydrogen induced cracking (cold cracking). SMAW is also highly portable, as it generates its own shielding gas, making it suitable for field applications where external gas cylinders might be impractical. However, SMAW typically produces slag, which requires cleanup between passes, and the weld pool can be fluid depending on the electrode type, demanding skilled operation. Specialized electrodes, such as Supermissileweld®, are designed for problem steels like high carbon steels, offering exceptional strength and crack resistance.

 

Gas Metal Arc Welding (GMAW) / MIG Welding

Gas Metal Arc Welding (GMAW), commonly known as MIG welding, uses a continuously fed wire electrode and an external shielding gas to protect the weld pool. For high carbon steels, MIG welding is a popular choice due to its low-hydrogen nature and versatility. The process allows for good control over heat input, which is vital for high carbon steel to prevent excessive hardening and cracking.

When MIG welding high carbon steels, it is generally recommended to reduce the heat input by using lower welding currents and voltage settings, and to consider a smaller wire diameter.  However, extreme caution must be exercised when trying to reduce heat input by lower amperage and voltage because that generally means slower travel speeds.  When determining heat input it is critical to calculate the heat input, not just change amps and volts. Pulse MIG welding is an advanced technique that offers even better control over heat input and reduces spatter, making it particularly beneficial for welding thinner sections of high carbon steel. While solid wires like ER70S-6 are commonly used as low-hydrogen filler wires for carbon steel , flux-cored wires can also be advantageous in some applications, offering better out-of-position performance, though they produce more slag.

 

Gas Tungsten Arc Welding (GTAW) / TIG Welding

Gas Tungsten Arc Welding (GTAW), or TIG welding, is a precise welding process that uses a non-consumable tungsten electrode to generate the arc, with a separate filler metal typically added manually. An inert shielding gas (usually argon or helium) protects the weld area from contamination. TIG welding is often preferred for thinner pieces of high carbon steel where precise heat control is critical to avoid excessive heat input, which can lead to brittleness and cracking.

The GTAW process offers superior control over the weld pool and heat input, allowing for very clean, precise, and high-quality welds. It produces no slag, eliminating the need for post-weld cleaning. For carbon steels, triple deoxidized mild steel wires like ER70S-2 are suitable for defect-free weld deposits, even on rusty or dirty surfaces. A key strategy for TIG and all other welding processes in welding high carbon steel is undermatching the filler metal, meaning using a filler rod one or two grades lower in strength than the base metal to enhance ductility and crack resistance. While TIG is excellent for quality, it is generally slower and requires a higher skill level than MIG or SMAW.

 

Other Welding Processes

While SMAW, GMAW, and GTAW are the most common, other processes can be used for high carbon steel in specific contexts. Oxyacetylene welding, Thermit welding, and Resistance welding are also listed as applicable processes, often for repair purposes. Oxy-acetylene welding, for instance, requires a carburizing flame for strong, defect-free welds on high carbon steel, and no flux is needed.

 

Filler Metal Selection

Beyond the general recommendation of low-hydrogen filler metals, specific strategies for filler metal selection are crucial when dealing with the unique properties of high carbon steel.

 

Matching vs. Undermatching Strength

When welding high strength steels, including high carbon steels, the choice between matching and undermatching the filler metal strength to the base metal’s strength is a critical engineering decision.

 

• Matching Strength: In many general welding applications, the filler metal’s minimum specified tensile strength is chosen to be equal to or greater than that of the base metal. This ensures the weld is as strong as or stronger than the parent material. This approach is often required for complete joint penetration welds where full strength is needed across the entire joint.

 

• Undermatching Strength: For high carbon steels, however, undermatching filler metal strength is frequently preferred. This means selecting a filler metal with a lower tensile strength than the base metal. The primary benefit of undermatching is significantly improved ductility and better crack prevention. A harder weld joint is more prone to cracking, so a softer, more ductile undermatching filler metal can accommodate the stresses of welding and cooling more effectively, leading to a more crack-resistant joint. While the overall strength of the joint might be technically lower than if a matching filler were used, the enhanced crack resistance often results in a “stronger” and more reliable weld in practice. This strategy is particularly useful when joining high strength steels and for certain fillet welds or partial joint penetration welds.

 

Low-Hydrogen Electrodes: The First Line of Defense

As previously discussed, hydrogen is a major culprit in cold cracking. Therefore, low-hydrogen filler metals are the cornerstone of successful high carbon steel welding. These electrodes are designed to contain minimal diffusible hydrogen (e.g., H4 or H8 designations indicate less than 4 or 8 ml of hydrogen per 100g of weld metal, respectively). Proper storage and handling of these electrodes are essential to maintain their low-hydrogen properties; they must be kept dry, often in heated rod ovens, to prevent moisture pickup. 

 

Austenitic Stainless Steel Filler Metals (e.g., 309-type)

In specific cases, 309-type austenitic stainless steel welding consumables are recommended for welding high carbon steels. This might seem counterintuitive, as stainless steel is a different alloy family. However, these filler metals offer distinct advantages:

• Low Diffusible Hydrogen: Austenitic stainless steel filler metals inherently have very low diffusible hydrogen content, which directly contributes to preventing hydrogen-induced cracking in the high carbon steel base material.

 

• Dissimilar Metal Joining: These filler metals are commonly used for welding dissimilar metals, including joining stainless steels to carbon steels, making them versatile for repair or fabrication involving mixed materials. The duplex weld deposit produced by these alloys contains delta ferrite, which minimizes cracking issues by absorbing carbon and tramp elements.

 

Nickel-Based Filler Metals

Nickel-based filler metals can also be a valuable option for welding high carbon steels, particularly in situations involving dissimilar metal joining or when specific properties like high strength and corrosion resistance at elevated temperatures are required. For example, Weldcote 82 (a nickel alloy TIG wire) is used for surfacing steel and joining stainless steels to nickel alloys to carbon steels. These alloys offer good corrosion and oxidation resistance, high-temperature strength, and can produce sound joints. Their versatility makes them a consideration for repairs or applications where the carbon content is unknown.

 

Welding Techniques for High Carbon Steel

Beyond selecting the right process and filler metal, the actual execution of the weld plays a crucial role in managing the challenges of high carbon steel. Specific techniques can significantly influence heat input, cooling rates, and stress distribution.

Controlling Heat Input

Precise control of heat input is paramount to minimize the size of the Heat Affected Zone (HAZ) and prevent the formation of brittle microstructures like martensite. High heat input can lead to grain coarsening in the HAZ, reducing toughness and increasing crack risk. Strategies include using lower welding currents and voltages, maintaining a short arc length, and increasing travel speed. Avoiding excessive weaving also helps concentrate the heat and prevent widespread thermal changes. For multi-pass welds, using smaller beads with multiple passes is preferable to large single passes, as subsequent passes can temper previously welded areas, improving HAZ toughness.

 

Travel Speed and Arc Length

Maintaining consistent and appropriate travel speed is essential. Welding too slowly can lead to excessive heat buildup, a larger HAZ, and increased distortion. Conversely, welding too fast can result in insufficient penetration, lack of fusion, and inadequate shielding, leading to porosity or undercut. A steady travel speed ensures uniform weld bead appearance and consistent gas coverage. Similarly, maintaining a short arc length is crucial, especially with SMAW low-hydrogen electrodes. A longer arc length can lead to arc instability, increased spatter, and porosity due to insufficient shielding. It can also prematurely destroy electrode coatings, further compromising shielding.

 

Backstepping Technique

The backstepping technique is a valuable method for managing heat distribution and reducing residual stresses, particularly in longer welds or those with high restraint. In this technique, the welder welds a short segment in one direction, then returns to a point just before the start of the last weld and begins the next pass, stopping at the beginning of the previous one. Essentially, the heat from the subsequent weld pass serves as a form of stress relieving, preheating the area ahead of the current weld and slowing its cooling. This helps prevent cold cracking by allowing more time for hydrogen to diffuse and promoting a more ductile microstructure.

 

Multi-pass Welding

For thicker sections of high carbon steel, multi-pass welding is often preferred over attempting a single, large pass. While a single pass might seem faster, it typically requires very high heat input, which can lead to a large, brittle HAZ and increased residual stresses. Multi-pass welding, by depositing smaller weld beads in successive layers, allows for better control of heat input and thermal cycles. Each subsequent pass reheats and tempers the previously deposited weld metal and HAZ, refining the grain structure and improving toughness. This approach helps to distribute stresses more evenly and reduces the overall risk of cracking.  However, keep in mind that a large numbrer of smaller passes with lead to more distortion that fewer larger passes, so there is a tradeoff. 

 

Successfully welding high carbon steel demands a comprehensive understanding of not just the material itself, but also the intricacies of welding processes and techniques. By carefully selecting the appropriate welding process (SMAW, GMAW, or GTAW) and employing advanced filler metal selection strategies like undermatching and utilizing low-hydrogen or austenitic stainless steel consumables, fabricators can significantly enhance weld integrity. Furthermore, optimizing welding techniques through meticulous heat input control, precise travel speed and arc length management, and strategic methods like backstepping and multi-pass welding, directly addresses the inherent challenges of these materials. In our next and final article of this 3-part series, we will delve into the critical aspects of quality control and advanced heat treatment procedures that are essential for ensuring the long-term reliability and performance of high carbon steel welds.