Summary of Key Points in Fastener Forging Processes

In fastener manufacturing, the forging process is known as the "strength shaper" — it reshapes the grain structure through metal plastic deformation, significantly improving the tensile strength, yield strength, and toughness of fasteners. It is a required process for high-strength, high-reliability fasteners (such as grade 8.8 bolts and above). Compared to machining, forging preserves the integrity of metal flow lines, increasing fastener load capacity by more than 30%, with material utilization rates of 85%-95%, offering both performance and economic advantages. To achieve high-quality forging results, precise control over the entire process from material selection to inspection is required. The following is a systematic elaboration from multiple dimensions.

Material selection is the foundation of the forging process, requiring consideration of both strength requirements and process compatibility. Ordinary strength fasteners (grades 4.8, 6.8) often use low-carbon steels such as Q235 and 10# steel. These materials have excellent plasticity and are easy to forge, making them suitable for cold or warm forging processes. Medium-to-high strength products (grades 8.8, 9.8) prioritize medium-carbon steels or alloy structural steels such as 45 steel and 40Cr, with carbon content strictly controlled between 0.37%-0.45% to ensure hardenability. For high-strength fasteners of grade 10.9 and above, alloy structural steels such as 35CrMo and 42CrMo are required. Their chromium and molybdenum content significantly improves high-temperature strength and toughness, while sulfur and phosphorus impurity content must be controlled below 0.035%. Beyond the base material, material pretreatment is also critical, focusing on three aspects: first, purity control — base material inclusion content should be ≤0.02% to avoid forming crack sources during forging; second, surface cleaning — removing scale, rust, and oil by shot blasting or pickling to prevent impurities from being pressed into the forging; third, grain refinement — normalizing alloy steel at 850°C-900°C for 1-2 hours to refine the initial grain structure and improve forging plasticity.

The choice of forging process type must be precisely matched to fastener specifications and performance requirements. Hot forging is performed above the material's recrystallization temperature, with heating ranges of 850°C-1200°C for carbon steel and 900°C-1250°C for alloy steel. Its core advantages are good plasticity and low deformation resistance, making it especially suitable for large-size high-strength bolts above M16 and large nuts. When applying hot forging, the heating rate should be controlled to not exceed 200°C/h to avoid overheating, and timely normalization after forging is required to eliminate internal stress. Unlike hot forging, cold forging is performed at room temperature to 200°C. It offers high precision (tolerance ±0.05 mm), smooth surfaces, and no scale, and is mostly used for precision fasteners and micro screws below M12. Before cold forging, the material requires annealing softening treatment at 650°C-700°C to improve plasticity, while the die must be wear-treated to extend service life. Warm forging lies between hot and cold forging, with temperatures ranging from 200°C to the material's recrystallization temperature — typically 600°C-800°C for carbon steel. This process combines plasticity and precision, reduces die wear, and is suitable for medium-size high-strength fasteners of M8-M16. In warm forging, temperature must be controlled within ±20°C to avoid local overheating, and a lubrication scheme of graphite plus oil is needed to reduce the friction coefficient.

Precise control of key process parameters is the core link determining forging quality. Regarding heating parameters: for hot forging, carbon steel must be heated to Ac3 + 30°C-50°C (for 45 steel, Ac3 is 780°C, corresponding to 810°C-830°C), while alloy steel requires Ac3 + 50°C-80°C. Holding time is calculated as "30 minutes per 10 mm of thickness" to ensure complete heating of the material core. During heating, overheating must be strictly avoided — temperatures above 1250°C easily cause coarse grains and oxidation burning, leading to forging cracking. Therefore, furnace temperature must be monitored in real time with thermocouples. Control of deformation parameters is equally critical. Total deformation should be maintained at 30%-70%: too little deformation cannot sufficiently refine grains, while too much easily causes stress concentration. In stepwise forging, each step's deformation should not exceed 40%, with annealing inserted between steps to release stress. Deformation speed must also match the process type: 10-50 mm/s for hot forging, 5-20 mm/s for cold forging. Excessive speed easily causes uneven metal flow and folding defects, while too slow speed reduces production efficiency and may increase oxidation loss in hot forging. Die and lubrication selection directly affects forging quality and production efficiency. For hot forging, choose H13 hot-work die steel; for cold forging, choose Cr12MoV cold-work die steel. Die hardness must reach HRC50 or above. Lubrication schemes must match the process: hot forging uses glass lubricants or graphite powder; cold forging uses extreme-pressure cutting oil plus graphite, controlling the friction coefficient between 0.05 and 0.1. The cooling process must be adjusted based on material characteristics. After hot forging, carbon steel uses air cooling (cooling rate ≤50°C/h), while alloy steel uses sand cooling or furnace cooling (cooling rate ≤30°C/h) to avoid rapid cooling that would form martensite and cause cracking. After cold forging, stress relief annealing at 600°C-650°C for 1 hour is needed to eliminate cold working stress, laying the foundation for subsequent heat treatment.

Quality inspection is a key step in avoiding forging defects and ensuring product reliability, covering multiple dimensions: appearance, dimensions, internal quality, and mechanical properties. Appearance inspection must ensure the forging has no cracks, folds, pressed-in scale, or excessive flash. Surface micro-cracks can be detected visually or by magnetic particle inspection (for ferromagnetic materials). Surface roughness should be controlled to Ra ≤ 3.2 μm, with no burrs or edge collapse on preformed threads. Dimensional accuracy inspection must follow GB/T 9145 standards, using calipers, projectors, or CMMs to measure critical dimensions such as bolt shank diameter and head thickness. Forging weight deviation must be controlled within ±2% to avoid overweight caused by uneven metal flow. Internal quality inspection mainly includes metallographic structure analysis and internal defect detection. After hot forging, grain size must reach grade 6 or above (per GB/T 6394), with no coarse grains, Widmanstätten structure, or other undesirable structures. After cold forging, grains must be uniformly refined without excessive concentration of deformation texture. Internal defects such as shrinkage cavities and porosity must have an equivalent defect size ≤ Φ2 mm by ultrasonic testing (UT). High-strength fasteners require 100% inspection, while ordinary parts have a sampling rate of not less than 30%. Mechanical property sampling must be targeted: tensile testing is required for tensile strength and yield strength — for example, grade 10.9 bolts require tensile strength ≥1040 MPa and yield strength ≥940 MPa. Fasteners used in low-temperature conditions require impact testing, with impact energy Akv ≥27 J at -40°C to avoid low-temperature brittle fracture.

During production, targeted solutions for common defects must be developed. Cracks are mostly caused by uneven heating, excessive deformation, or too rapid cooling. Solutions include optimizing the heating curve to ensure temperature uniformity, using stepwise forging to reduce single-step deformation, and selecting appropriate cooling methods based on material characteristics. Forgings that have already cracked must not be allowed to proceed to subsequent processes. Folds are associated with inconsistent metal flow direction and excessively small die radii. Solutions include adjusting die design (radius ≥3 mm), optimizing forging sequence to ensure unidirectional metal flow, and avoiding interweaving of metal flow lines from different directions. Coarse grains are usually caused by excessively high heating temperatures or too long holding times. Solutions include strictly controlling furnace temperature fluctuations within ±10°C, reasonably shortening holding time, and adding grain-refining alloying elements such as vanadium and titanium to improve material properties. Dimensional deviation is mostly caused by die wear or improper deformation control. Solutions include regularly inspecting dies to ensure wear ≤0.03 mm and using closed-loop control systems to precisely control deformation parameters, ensuring dimensional accuracy stability.

As manufacturing transforms toward high-end, intelligent, and environmentally friendly processes, fastener forging technology is also continuously upgrading. Intelligent upgrading is an important trend. CNC forging equipment is gradually becoming widespread, monitoring key parameters such as temperature, pressure, and deformation in real time, combined with AI algorithms to optimize process parameters, effectively reducing human error and improving product consistency. Precision forging technology is being widely promoted. Near-net shape processes reduce subsequent machining. For example, precision cold-forged threads can achieve grade 6H accuracy, greatly improving production efficiency and material utilization. Environmentally friendly improvements are imperative. Electric heating replaces traditional coal heating to reduce exhaust emissions. Plant-based and other environmentally friendly lubricants gradually replace traditional lubricants, reducing environmental pollution. Material innovation meets high-end requirements. For lightweight requirements in new energy vehicles and aerospace, forging processes for titanium alloy and composite material fasteners are continuously advancing, with a focus on solving the challenge of controlling deformation stability at high temperatures, providing more reliable connection solutions for high-end equipment.

The core of the forging process is to achieve dual improvements in fastener performance and precision through precise control of metal plastic deformation. The key points above cover the entire process from material selection to inspection, conforming to industry standards and practical production scenarios. For fastener professionals, deeply mastering these core points can effectively reduce production defect rates, improve product reliability, and help companies gain a competitive advantage in the high-end manufacturing market.