Investigation of the parameters causing hardness change in screw thread profile in bolt production with plastic deformation

Fasteners are critical components in modern engineering applications. Fasteners are produced with many different production methods in today's technology. In this context, the ISO 898-1:2013 standard is an international reference point for fasteners. It specifies safe and consistent performance ranges for bolts at ambient temperatures ranging from 10-30 °C. This standard defines the properties of fasteners, such as durability, hardness and mechanical strength, and ensures production in accordance with the areas of use. In particular, ensuring that the hardness difference between the upper and lower points of the tooth profile is 30 HV with the hardness range of 320-380 HV required by the ISO 898- 1:2013 standard causes many changes in the process. In this thesis, the conformity of the Vickers hardness distribution in the teeth to the standard was analyzed after tooth profile formation on steels produced from raw materials such as 42CrMo4+QT, 33MnCrB5-2+QT etc. The factors affecting the hardness difference in the teeth were examined in detail and ways to enter the range required by the standard were sought. In the study, 42CrMo4+QT, 33MnCrB5- 2+QT, 34CrNiMo6 etc. raw materials were confirmed and the experiment was analyzed under 4 main groups. In the first group, cutting and rolling process differences, in the second and third groups, machine parameters and in the fourth group, the effects of the raw material on the process were investigated. Vickers hardness distribution measurements on the threads of the bolts were determined according to the raw material, machine parameter and manufacturing type by threading on the sample parts. In bolts produced by plastic deformation method, after all the production processes required by the standard are completed, the hardness difference between the top and bottom of the thread should be max. 30 HV and the thread hardness range should be 320-380 HV according to ISO 898-1: 2013 standard. The parameters that cause this hardness difference observed after manufacturing and the reasons for the hardness difference were analyzed. The test results of the experimental studies were obtained in ISO/IEC 17025:2017 accredited laboratory. According to the test results, it was determined that the most effective parameter to enter the hardness range specified by the ISO 898- 1:2013 standard is the raw material. In the test stages, it was observed that 33MnCrB5-2+QT raw material was able to enter the specified standard range. KEYWORDS: Bolt, Plastic Deformation, Thread Profile, Hardness, Strength, Strain Hardening, Dislocations.