A study of high-strength bolts after dephosphoring

Shao-Yi Hsia,Jung-Chi (Robin) Chao,Yu-Tuan Chou

doi:10.1177/1687814016637317

Abstract

A wide variety of fasteners are produced, including those for the automobile industry, household electrical appliances industry, architectural engineering, and even the aviation industry. The effects of the high-tensile bolt dephosphoring process on the entire fastener manufacturing process and its organizational characteristics and mechanical properties are analyzed and discussed in this study. Our experimental results reveal that the bolt dephosphoring process must be completed before heat treatment, which can be confirmed with a dephosphoring reagent or metallographic observation. Once bolt heat treatment is completed, bolts without dephosphoring appear to be coated with δ ferrite (delta ferrite) composed of a phosphate coating and a phosphatizing coating, which are not easily removed. Heat treatment with phosphorus results in grain boundary segregation, causing embrittlement and a reduction in lattice bonding forces and resulting in a high risk of fracturing when bolts are used in high-temperature environments or undergo multiaxial stresses.

Highlights

Improvements in fabrication equipment and the broad application of simulation software have allowed Taiwan’s fastener production technology and industry to mature
Samples A and B without dephosphoring but directly electroplated after heat treatment had a white phosphorus layer under the electroplating layer (Figure 5), and the phosphatizing layer was not removed in the acid pickling electroplating process but covered by the electroplating layer (Figure 6)
Samples C and D (Figure 7) did not have phosphate coatings or phosphatizing layers after heat treatment and electroplating. This reveals that removing phosphorus from screws before heat treatment effectively prevents phosphate coatings and phosphatizing layers from forming

Summary

Introduction

Improvements in fabrication equipment and the broad application of simulation software have allowed Taiwan’s fastener production technology and industry to mature. In addition to CL10.9 and CL12.9 high-tensile bolt materials in the production process being regulated, Volkswagen[7] and Ford[8] require the removal of surface phosphorus coatings on high-tensile bolts before heat treatment to avoid brittle fractures. By tracing the production process, it was discovered that phosphorus was not removed from the bolt as required before heat treatment and that hydrogen degassing occurred after surface treatment.

Results

Conclusion