Class 10.9 bolts are widely‑used high‑strength hexagonal fasteners. Thanks to their high strength and hardness, they are frequently applied in working conditions such as precision mechanical equipment and tooling dies. What is the standard hardness range for Class 10.9 bolts? This article provides an analysis based on industry references for your information.
First, let us look at the hardness indicators of Class 10.9 bolts. According to the national standard GB/T 3098.1 for fasteners, the core Vickers hardness of Class 10.9 bolts ranges from HV320‑380, corresponding to Rockwell hardness HRC32‑39. This hardness range shall be achieved through reasonable quenching and tempering heat‑treatment processes. In actual production, bolts are austenitized and then immersed in quenching oil for quenching, followed by high‑temperature tempering. The holding time and temperature parameters of quenching and tempering play a decisive role in the final hardness and comprehensive mechanical properties.
Next, we dive into practical manufacturing process cases for Class 10.9 bolts.
Process Case 1
Labelled as the 35CrMo process scheme, while 40Cr alloy steel is actually adopted for bolt production. The bolt measures 22 mm in diameter and 114 mm in length, used for truck‑wheel fastening. After quenching and tempering heat treatment, its hardness shall reach HV320‑380 (HRC32‑39), with a minimum tensile strength of 1040 MPa. The ideal metallographic structure is tempered sorbite.
Process Case 2
Test Scheme 1: Hot forging forming followed by machining, with heat treatment carried out in a mesh‑belt furnace. The temperature settings of Zone 1‑4 for quenching are 860‑870‑870‑860 ℃ respectively, with total quenching holding time of approximately 90 minutes, followed by oil quenching for cooling. The hardness after quenching reaches HRC49‑52. Tempering temperature for Zone 1‑3 is uniformly set at 510 ℃, and the hardness after tempering reads HRC32‑35. Inspection results show that the tensile strength of products under this process fails to meet Class 10.9 requirements, hence the process is deemed unqualified.
Test Scheme 2: Production route consisting of annealing, phosphating and cold extrusion forming. Quenching temperatures of Zone 1‑4 in the mesh‑belt furnace are 860‑870‑870‑860 ℃, quenching holding time around 90 minutes, oil quenching cooling; post‑quenching hardness HRC49‑52. Two sets of tempering parameters are compared: Group 1: Tempering temperature of 480 ℃ for all Zone 1‑3. Hardness after tempering: HRC34‑38, measured tensile strength: 1005 MPa. A small amount of undissolved ferrite is observed in metallographic inspection. Group 2: Tempering temperature of 460 ℃ for all Zone 1‑3. Hardness after tempering: HRC36‑40, measured tensile strength: 960 MPa, which does not satisfy the minimum tensile‑strength requirement for Class 10.9.
It can be seen that even if hardness falls within the target range, products are still non‑conforming if tensile strength fails requirements. Minor ferrite will degrade mechanical performance of bolts and shall be strictly controlled under working conditions with high fatigue requirements.
In summary, the qualified core‑hardness range for Grade 10.9 bolts is HV320‑380, corresponding to HRC32‑39. General reference heat‑treatment parameters: quenching at 850 ℃ with 60‑minute holding, carbon potential controlled at 0.45, isothermal graded quenching oil at 100 ℃ oil temperature, quenching holding for 30 minutes; tempering at 480 ℃ with 120‑minute holding. During production, hardness alone cannot be taken as the only criterion. Tensile strength, yield strength and other mechanical properties shall all comply with national standards. The metallographic structure should preferably be dominated by tempered sorbite. Non‑ideal structures such as ferrite should be minimized to avoid impairing load‑bearing and anti‑fatigue performance of bolts.
