Welding of Steels

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Introduction to Steel Classification

Almost 85% of the metal produced and used worldwide is steel. Steel, principally an alloy of iron and carbon, often contains additional metals such as manganese, chromium, nickel, and nonmetals including silicon, phosphorus, and sulfur. The diverse classification of steels—including structural steels, cast steels, stainless steels, tool steels, hot-rolled steel, reinforcing steel, and low-alloy high-strength steel—can make identification challenging. Steels are frequently named according to their principal alloying elements, such as carbon steel, chrome-manganese steel, or chrome-molybdenum steel.

Welding Low-Carbon and Low-Alloy Steels

Low-Carbon Steels

Low-carbon steels include those in the AISI series C-1008 to C-1025. These steels contain 0.10-0.25% carbon, 0.25-1.5% manganese, maximum 0.4% phosphorous, and maximum 0.5% sulfur. As the most widely used materials for industrial fabrication and construction, these steels can be readily welded using arc, gas, and resistance welding processes.

Low-Alloy High-Strength Steels

The low-alloy high-strength steels constitute the majority of the remaining steels in the AISI designation system. These steels require E-80XX, E-90XX, and E-100XX classes of covered welding electrodes. This category includes low-manganese steels, low-to-medium nickel steels, low nickel-chromium steels, molybdenum steels, chromium-molybdenum steels, and nickel-chromium-molybdenum steels.

Alloys in the AISI series 2315, 2515, and 2517 contain 0.12-0.30% carbon, 0.40-0.60% manganese, 0.20-0.45% silicon, and 3.25-5.25% nickel. Preheating is generally unnecessary for carbon content below 0.15%, except for extremely heavy sections. When carbon exceeds 0.15%, preheating up to 260°C may be required, depending on material thickness.

Electrode Selection for Low-Carbon and Low-Alloy Steels

For shielded metal arc welding (SMAW), electrodes described in AWS specification A5.1 are suitable for mild and low-alloy steels. The E-60XX and E-70XX electrode classes produce sound welds in these steels, with yield strengths that exceed those of the base metals. E-60XX electrodes are appropriate for steels with yield strength below 350 MPa, while E-70XX electrodes should be used for steels with yield strength below 420 MPa. Low-hydrogen electrodes are recommended, especially for heavier materials or restrained joints.

When welding low-alloy high-strength steels, most E-80XX and higher-strength electrodes feature low-hydrogen coverings, with the E-XX10 class being the sole exception. The AWS 5.5 specification for low-alloy steel-covered arc welding electrodes includes various strength levels and electrode types such as E-8010, E-XX15, E-XX16, and the widely used E-XX18 classes.

Proper electrode selection requires knowledge of both the base metal's mechanical properties and composition to ensure appropriate matching. The E-8010 electrodes, which lack low-hydrogen coverings, are specifically designed for pipe welding. These cellulose-covered electrodes offer deep penetration characteristics ideal for cross-country pipeline applications. Their effectiveness relies on the relatively thin steel pipe sections, high welding currents, and the aging period before service that allows hydrogen to escape.

Welding Medium-Carbon Steels

Medium-carbon steels encompass the AISI series C-1020 to C-1050. Similar in composition to low-carbon steels, they contain higher carbon (0.25-0.50%) and manganese (0.60-1.65%) content.

Due to the increased carbon and manganese, low-hydrogen electrodes are strongly recommended, particularly for thicker sections. Preheating at 150-260°C may be necessary, and post-weld heat treatment is often specified to relieve stress and reduce hardness caused by rapid cooling. With proper precautions, medium-carbon steels can be successfully welded using all previously mentioned processes.

Welding High-Carbon Steels

High-carbon steels include those in the AISI series C-1050 to C-1095, with carbon content ranging from 0.30-1.00% and composition otherwise similar to medium-carbon steels.

Welding high-carbon steels requires special precautions. Low-hydrogen electrodes are essential, and preheating at 300-320°C is necessary, especially for heavier sections. Post-weld heat treatment, either stress relieving or annealing, is typically specified. These steels can be welded using the same processes mentioned earlier, provided these precautions are observed.

Welding Specialty Steel Alloys

Low-Nickel Chrome Steels

This group includes AISI 3120, 3135, 3140, 3310, and 3316 steels, containing 0.14-0.34% carbon, 0.40-0.90% manganese, 0.20-0.35% silicon, 1.10-3.75% nickel, and 0.55-0.75% chromium.

Thin sections with lower carbon content can be welded without preheating. However, material with approximately 0.20% carbon requires 100-150°C preheat, while higher carbon content necessitates preheating up to 320°C. Post-weld stress relief or annealing is essential.

Low-Manganese Steels

This category encompasses AISI types 1320, 1330, 1335, 1340, and 1345, with 0.18-0.48% carbon, 1.60-1.90% manganese, and 0.20-0.35% silicon.

Preheating is not required for steels with low carbon and manganese content. As carbon approaches 0.25%, 120-150°C preheat becomes advisable and mandatory at higher manganese levels. Thicker sections should be preheated to twice these temperatures. Post-weld stress relief treatment is recommended.

Low-Alloy Chromium Steels

This group includes AISI types 5015 to 5160 and electric furnace steels 50100, 51100, and 52100, containing 0.12-1.10% carbon, 0.30-1.00% manganese, 0.20-1.60% chromium, and 0.20-0.30% silicon.

Lower carbon content steels in this category can be welded without special precautions. As carbon and chromium increase, hardenability rises, potentially requiring preheating up to 400°C, particularly for heavy sections.

Advanced Welding Process Considerations

Submerged Arc Welding

When using submerged arc welding, electrode composition should match the base metal, and the flux should neither add nor detract elements from the weld metal. Generally, lower preheat temperatures can be used with this process due to its higher heat input and slower cooling rates. To ensure low hydrogen content, the flux must be dry, and both electrode and base metal must be clean.

Gas Metal Arc Welding

For gas metal arc welding (GMAW), electrodes should match the base metal, and shielding gas selection should prevent excessive weld metal oxidation. Preheating requirements are similar to those for shielded metal arc welding due to comparable heat input.

Flux-Cored Arc Welding

With flux-cored arc welding, the deposited weld metal should match the base metal being welded. Preheat requirements are similar to those for gas metal arc welding.

Welding Dissimilar Steels

When welding low-alloy high-strength steels to lower-strength grades, electrodes should be selected to match the lower-strength steel. However, the welding procedure, including heat input and preheating, should be suitable for the higher-strength steel.