Release time：2020-01-16 18:44 Browse：
Regarding whether extra-long and special-length 6.8-grade screws require heat treatment, under normal circumstances, 6.8-grade screws are not mandatory to undergo heat treatment. Their strength can be achieved through cold working (such as cold heading or thread rolling) or appropriate material selection. However, the need for heat treatment is also related to specific materials, processing techniques, and screw specifications.
Below is a quick comparison to help you understand the typical treatment methods for screws of different strength grades:
| Strength Grade | Typical Material | Common Treatment Method | Usually Requires Heat Treatment? |
|---|---|---|---|
| Grade 4.8 | Low Carbon Steel (e.g., 1010) | Cold Working | No |
| Grade 6.8 | Medium Carbon Steel (e.g., 35# Steel) | Cold Working (primary) or may involve heat treatment in some cases | Usually No |
| Grade 8.8 | Medium Carbon Steel, Low Carbon Alloy Steel | Quenching and Tempering (quenching + high-temperature tempering) | Yes |
| Grade 10.9 | Medium Carbon Alloy Steel | Quenching and Tempering | Yes |
| Grade 12.9 | Alloy Steel | Quenching and Tempering | Yes |
Common Methods for Achieving Grade 6.8 Screws
Grade 6.8 screws require a tensile strength of no less than 600 MPa and a yield strength of no less than 480 MPa. There are two main approaches to achieving this:
Cold Working (Cold Hardening): This is the most common method. For carbon steel materials, processes such as cold heading and thread rolling induce plastic deformation, leading to work hardening and thereby enhancing strength. Many carbon steel materials (e.g., ML35, 35# steel) used for Grade 6.8 screws can meet performance requirements through cold working without additional heat treatment.
Heat Treatment (Quenching and Tempering): If the screw dimensions are particularly large (e.g., "extra-long and special-length" as mentioned by the user), or if the raw material struggles to fully meet Grade 6.8 performance indicators after cold working, manufacturers may opt for heat treatment (quenching and tempering) to ensure mechanical properties. However, this is not the standard practice.
Potential Impact of "Extra-Long and Special-Length"
The "extra-long and special-length" aspect you mentioned requires special attention:
Size Effect: For screws that are very long or have a large shank diameter, ensuring uniform cold work hardening effects across the entire length or core during cold working becomes significantly more challenging. If the core strength is insufficient, it may affect overall performance.
Processing Stress: Extra-long screws may accumulate greater internal stresses during cold working. Without subsequent stress relief annealing, this could impact dimensional stability or increase the risk of delayed cracking.
How to Confirm Whether Heat Treatment Is Required
To accurately determine whether your extra-long and special-length Grade 6.8 screws have undergone heat treatment, the most reliable methods are:
Review technical documentation or material certificates provided by the supplier: These documents typically specify the screw's production process and heat treatment status.
Consult directly with the supplier or manufacturer: Clearly inquire about the production process for this specific product and whether quenching and tempering was performed to ensure performance.
Perform necessary testing: If conditions allow, hardness testing (the hardness range for Grade 6.8 screws is approximately HRB 89 to HRC 22) or metallographic analysis (observing whether the internal structure shows flow lines from cold working or tempered sorbitel from quenching and tempering) can assist in judgment.
Summary
Ordinary Grade 6.8 screws typically rely on cold working to achieve strength requirements and generally do not require heat treatment.
For "extra-long and special-length" Grade 6.8 screws, to ensure performance uniformity and stability, there is a possibility that heat treatment (quenching and tempering) might be used. However, this is not absolute and depends on the manufacturer's specific process choices.
The safest approach is to confirm through official technical documentation or by directly contacting the supplier.
