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Zhejiang Donghe Machinery Technology Co., Ltd., located in Jiaxing City, is a high-tech enterprise specializing in the R&D and manufacturing of high-performance fasteners. Established in 2009, our facility operates with over 100 sets of advanced machinery and a production capacity of 8,000 tons. Holding ISO9001:2000 certification and self-operating export rights, we produce components adhering to GB, DIN, ANSI, and B standards. For specialized industrial applications, we engineer the non standard screw to meet precise client specifications. In heavy-duty structural assemblies, the shear strength of these fasteners is the primary safety metric. Achieving the required mechanical properties necessitates a rigorous heat treatment process for industrial screws to modify the internal martensitic structure of the steel alloy.
Metallurgical Transformation and the Hardness-Ductility Balance
The shear strength of a non standard screw is fundamentally linked to its core hardness and grain structure refinement. During the austenitizing phase, the fastener is heated above its critical temperature (typically 850°C to 900°C for medium carbon steels) to dissolve carbides into the iron matrix. A rapid quenching process for heavy-duty fasteners in oil or polymer media transforms the austenite into martensite, a highly stressed and hard crystalline structure. However, Why is tempering necessary for non standard screws? Without tempering, the material remains excessively brittle, leading to sudden shear failure under lateral loads. By reheating the component to a specific tempering temperature, we reduce internal stresses and enhance the tensile strength and toughness of screws, ensuring the fastener can withstand dynamic shearing forces without fracturing.
Case Hardening vs. Through Hardening for Shearing Resistance
Depending on the application, a heavy-duty non standard screw may require different hardening depths. Case hardening of non-standard fasteners, such as carburizing, creates a wear-resistant surface (HRC 58-62) while maintaining a ductile core. This is vital for preventing thread galling in screws used in moving assemblies. Conversely, for structural bolts where lateral loads are distributed across the entire cross-section, through hardening of carbon steel screws is mandatory to ensure uniform shear resistance. The material selection for high-strength screws, such as SCM435 or 40Cr, determines the hardenability limit. Failure to achieve a 90 percent martensite transformation at the center of the shank results in a "soft core," which significantly lowers the allowable shear stress of fasteners in critical infrastructure.
Impact of Quenching Media on Dimensional Stability
The dimensional accuracy of non standard screws can be compromised during thermal cycling. How to prevent screw warping during heat treatment involves selecting the correct quenching medium and agitation rate. Water quenching provides the highest cooling rate but increases the risk of quench cracks and distortion in complex geometries. For customized non-standard fasteners, oil quenching is often preferred to ensure a more moderate cooling curve. This maintains the Ra surface finish of screw threads and prevents the formation of micro-cracks that act as stress concentrators. Furthermore, decarburization control in fastener manufacturing is strictly monitored using protective atmospheres (such as endothermic gas) to prevent the loss of surface carbon, which would otherwise lead to a "soft skin" and reduced fatigue life.
| Mechanical Property | 8.8 Grade Standard | Donghe High-Strength Customization |
| Tensile Strength (MPa) | 800 - 830 | 1040 - 1220 (ASTM A354) |
| Yield Strength (MPa) | 640 - 660 | 940 - 1100 |
| Core Hardness (HRC) | 22 - 32 | 33 - 39 (Controlled) |
| Elongation (%) | Min 12 | Min 14 |
| Shear Strength (MPa) | Approx. 480 | > 620 (Tested) |
Quality Validation and Fastener Testing Standards
To ensure long-term development and reliability, Zhejiang Donghe Machinery Technology utilizes a full suite of inspection equipment. Testing the shear strength of non standard screws involves double-shear testing protocols where the fastener is subjected to perpendicular loads until rupture occurs. Why ISO9001:2000 is critical for fastener production relates to the traceability of the heat treatment batch records, ensuring that every quenching and tempering cycle is documented. For screws exported to Europe and the United States, we also perform Charpy V-notch impact tests to verify low-temperature toughness. By adhering to the philosophy of "pragmatism and efficiency," our team ensures that every custom non-standard screw provides the technical support and structural safety required for geology, mining, and industrial machinery sectors.
Industrial Hardcore FAQ
Q1: What is the ratio between tensile strength and shear strength for a non standard screw?
A1: Generally, the shear strength is approximately 60 percent to 75 percent of the ultimate tensile strength (UTS). For a 10.9 grade screw, if the UTS is 1000 MPa, the shear strength is typically around 600-700 MPa.
Q2: Can I use a non standard screw without any heat treatment?
A2: For low-carbon steel (like Q235) in non-load-bearing applications, yes. However, for "heavy-duty" applications, untreated screws lack the yield strength to prevent permanent deformation under load.
Q3: How does "over-tempering" affect a fastener?
A3: Over-tempering occurs when the temperature is too high or the duration is too long. This results in an excessively soft screw with high ductility but insufficient tensile and shear strength to meet industrial standards.
Q4: What causes "Hydrogen Embrittlement" in heat-treated screws?
A4: This occurs during acid pickling or electroplating when hydrogen atoms penetrate the grain boundaries of high-strength steel (above HRC 39). It leads to sudden, brittle failure under static load and must be mitigated by "baking" the screws immediately after plating.
Q5: Why do some screws require induction hardening only on the tip?
A5: This is common for self-tapping non standard screws where the tip must be harder than the material being joined, while the shank remains ductile to resist snapping during installation torque.
Technical References
- ISO 898-1: Mechanical properties of fasteners made of carbon steel and alloy steel - Part 1: Bolts, screws and studs with specified property classes.
- ASTM F606: Standard Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners.
- DIN 267-27: Fasteners - Part 27: Steel fasteners with adhesive coating, technical specifications.
