DIN 17100 St37-2 Mild Steel Sheets: Weldability Analysis & Post-Weld Heat Treatment Guidelines

DIN 17100 St37-2 is a widely utilized non-alloy structural steel, renowned for its versatility and cost-effectiveness in various engineering and construction applications. As a mild steel sheet, its mechanical properties and chemical composition make it highly amenable to fabrication processes, particularly welding. Understanding the intricacies of its weldability and the necessity of proper post-weld heat treatment (PWHT) is crucial for ensuring the integrity, performance, and longevity of fabricated structures. This comprehensive guide delves into the metallurgical characteristics influencing St37-2's weldability, outlines common welding procedures, and provides essential guidelines for effective post-weld heat treatment.

Understanding DIN 17100 St37-2: Composition and Properties

St37-2, equivalent to S235JR in the EN 10025 standard, is classified as a low-carbon structural steel. Its chemical composition is carefully controlled to optimize a balance between strength, ductility, and weldability.

• Carbon (C): Typically ranging from 0.17% to 0.20%, the low carbon content is a primary reason for its excellent weldability. Lower carbon minimizes the risk of hardening and cracking in the heat-affected zone (HAZ).
• Manganese (Mn): Usually between 0.50% and 1.40%, manganese contributes to strength and toughness while also helping to deoxidize the steel during welding, improving bead quality.
• Silicon (Si): Present in small amounts (up to 0.50%), silicon acts as a deoxidizer and can slightly increase strength.
• Phosphorus (P) and Sulfur (S): These impurities are kept to minimal levels (typically <0.050%) as they can negatively impact toughness and lead to hot cracking during welding.

Mechanically, St37-2 offers a minimum yield strength of 235 MPa for thicknesses up to 16mm, and a tensile strength typically between 360-510 MPa. This combination of moderate strength and high ductility makes it ideal for general structural applications where ease of fabrication is paramount. The low carbon equivalent (CE) value for St37-2 further confirms its high resistance to cold cracking, a common issue in welding higher carbon steels.

Weldability Analysis of St37-2 Mild Steel Sheets

The weldability of DIN 17100 St37-2 is generally considered excellent, making it a preferred choice for many fabrication projects. Its low carbon content ensures that it can be welded without extensive preheating in most situations, especially for thinner sections.

Pre-Welding Considerations

• Surface Preparation: Before welding, surfaces should be thoroughly cleaned of rust, scale, oil, grease, paint, and other contaminants. These can lead to porosity, inclusions, or hydrogen embrittlement.
• Joint Design: Proper joint design (e.g., butt, lap, fillet welds) is crucial for achieving full penetration and minimizing distortion. Beveling may be necessary for thicker sections.
• Preheating: For St37-2, preheating is typically not required for thicknesses below 20-25mm unless welding in very cold environments (below 0°C), or for highly restrained joints. For thicker sections or complex fabrications, a modest preheat (e.g., 50-150°C) can help reduce thermal gradients and mitigate the risk of hydrogen-induced cracking.

Common Welding Processes and Filler Materials

St37-2 is compatible with most standard welding processes. The choice depends on the application, desired speed, and quality requirements.

• Shielded Metal Arc Welding (SMAW / MMA): This is a versatile and widely used process. Low hydrogen electrodes (e.g., E7018) are preferred to minimize hydrogen content in the weld metal and HAZ, reducing the risk of cold cracking. Rutile electrodes (E6013) are also common for general purpose welding due to their ease of use and good bead appearance.
• Gas Metal Arc Welding (GMAW / MIG/MAG): Offering high deposition rates and good arc control, GMAW is highly efficient for St37-2. Solid wires (e.g., ER70S-6) are typically used with a shielding gas mixture of argon and CO2 (MAG) or pure CO2. This process produces clean welds with minimal slag.
• Gas Tungsten Arc Welding (GTAW / TIG): GTAW provides superior quality, precision, and aesthetic appeal, especially for thinner gauges or critical applications. It uses a non-consumable tungsten electrode and filler rod (e.g., ER70S-2) with an inert shielding gas like argon.
• Submerged Arc Welding (SAW): For thicker sections and high-production welding, SAW offers very high deposition rates and deep penetration. It employs a continuously fed wire electrode covered by a granular flux, resulting in smooth, high-quality welds.

Regardless of the chosen process, maintaining appropriate interpass temperatures is crucial to prevent excessive heat buildup, which can lead to grain growth and reduced toughness, or too rapid cooling, which can induce stresses.

Post-Weld Heat Treatment (PWHT) Guidelines for St37-2

While DIN 17100 St37-2 is generally forgiving, post-weld heat treatment (PWHT) can be a critical step for certain applications, especially when dealing with thicker sections, highly restrained joints, or components subjected to dynamic or fatigue loading. The primary goals of PWHT are to relieve residual stresses, improve ductility and toughness, and refine the microstructure of the weld and HAZ.

Why PWHT is Necessary

• Stress Relief: Welding introduces significant thermal gradients, leading to residual stresses upon cooling. These stresses can reduce the material's load-bearing capacity, increase susceptibility to brittle fracture, and cause distortion. PWHT reduces these internal stresses.
• Microstructure Homogenization: The HAZ can experience grain coarsening or the formation of harder, less ductile phases. PWHT helps to temper these areas, promoting a more uniform and refined microstructure.
• Improved Mechanical Properties: Enhanced ductility, impact toughness, and fatigue resistance are common benefits of proper PWHT, making the component more reliable in service.
• Dimensional Stability: For precision components, stress relief helps prevent further distortion or creep during machining or long-term service.

When is PWHT Recommended for St37-2?

• Thick Sections: For plate thicknesses exceeding 25-30mm, especially in highly restrained joints.
• Critical Applications: Structures exposed to dynamic or cyclic loading, low temperatures, or corrosive environments.
• Regulatory Requirements: Certain codes and standards may mandate PWHT for specific applications, irrespective of material type.
• Complex Geometries: Components with abrupt changes in section or multiple intersecting welds.

PWHT Procedures and Parameters (Stress Relieving)

The most common PWHT for St37-2 is stress relieving. The key parameters include:

• Heating Rate: Control the heating rate to prevent excessive thermal gradients, typically not exceeding 220°C per hour per 25mm of thickness.
• Soaking Temperature: For St37-2, stress relieving is typically performed in the range of 550°C to 650°C. The exact temperature depends on the desired properties and specific standards. Higher temperatures within this range promote more effective stress relief but can also lead to grain growth if held for too long.
• Soaking Time: A general rule of thumb is 1 hour per 25mm of thickness, with a minimum of 30 minutes for thinner sections. For more precise control, soaking time can be adjusted based on the required stress reduction and material properties.
• Cooling Rate: Slow, controlled cooling is essential after soaking to prevent the re-introduction of thermal stresses. The cooling rate should generally be similar to the heating rate, especially below 400°C. Cooling in still air for smaller components or within the furnace for larger ones is typical.

Alternatives to Stress Relieving

While less common for St37-2 simply for stress relief, other heat treatments like normalizing (heating to above critical temperature and air cooling) or full annealing (slow furnace cooling) can be used to refine grain structure and improve mechanical properties more significantly, often after heavy cold working or for extreme applications. These processes are more energy-intensive and time-consuming.

Applications of DIN 17100 St37-2

Given its excellent balance of properties and ease of fabrication, DIN 17100 St37-2 is widely used across various industries. It is a cornerstone material for general structural fabrication, including:

• Construction: Beams, columns, frames, and other structural components in buildings and bridges.
• General Engineering: Machine parts, equipment frames, and components where moderate strength and good weldability are required.
• Automotive Industry: Chassis components, brackets, and other non-critical structural parts.
• Shipbuilding: Secondary structures and fittings.
• Pipelines: Components for non-pressure applications.

As a leading manufacturer based in Shanghai, China, Baobin Steel has over 30 years of industry experience and supplies high-quality carbon steel materials, including DIN 17100 St37-2, to over 100 countries. Our expertise ensures that our products meet the stringent requirements for weldability and performance, contributing to a customer satisfaction rate of 98%.

Conclusion

DIN 17100 St37-2 mild steel sheets offer an excellent combination of strength, ductility, and, critically, outstanding weldability. Its low carbon content simplifies most welding operations, often negating the need for extensive preheating for thinner sections. However, for heavier gauges, complex structures, or highly critical applications, thoughtful consideration of preheating and, more importantly, post-weld heat treatment, is paramount. Proper application of stress relieving can significantly improve the final component's mechanical properties, reduce residual stresses, and enhance overall structural integrity and service life. By adhering to best practices in both welding and PWHT, engineers and fabricators can fully leverage the inherent advantages of St37-2, ensuring robust and reliable constructions. Baobin Steel is committed to providing top-tier steel and metal materials that meet global standards for quality and performance.