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.
Introduction
Improvements in fabrication equipment and the broad application of simulation software have allowed Taiwan’s fastener production technology and industry to mature. Consequently, stable quality and yields allow fastener manufacturers in Taiwan to maintain a critical position in the global fastening supply chain. To respond to the ever-increasing level of quality demanded by international customers, fastener manufacturers must constantly make improvements leading toward zero defects/0 ppm by refining their processes and careful selection of equipment.1–3 The status of the fastener industry in Taiwan is therefore consolidated within the international supply chain.
High-tensile bolts are mostly designed as primary structural elements with a stress on safety. Nonetheless, everything has its service life and fracturing marks the end of the service life of a bolt. A bolt with a fracture can be dangerous and deadly, so it is especially important that bolt fractures not occur on equipment where accidents would result.
According to Lee’s China Times report on 15 May 2012, “Bolt fracture, exposed to wind and rain for two years, was the prime culprit for the Kuosheng Nuclear Power Plant disaster.” 4 The broken anchor bolts used for securing the bottom of the nuclear reactor to the steel-reinforced concrete substrate holding the weight of the reactor were the prime culprits for the incident of Kuosheng Nuclear Power Plant Unit 1. According to the cause analysis and safety evaluation report of the Atomic Energy Council, the anchor fracture imputed to improper installation of bolts and the exposure to wind and rain for 2 years during the construction of the power plant accelerated metal fatigue. “Environmental factors” were believed to have aggravated the situation. Certain nuclear reactor pressure components, after being hung in the containment building during construction in the 1970s, were not immediately covered for their protection, so the bottom of the nuclear reactor and the steel-reinforced concrete substrate were exposed to wind and rain for 2 years, causing stress corrosion. Improper installation that continuously expanded the cracks together with metal fatigue caused the collective fracturing of the bolts.
Tsai, the vice general manager of TG Co., Ltd, pointed out in Hot Galvanizing High-tensile Bolts 4 the commonness of rusting in steel and steel structures, especially those without paint. Steel rusting (corrosion) is an electrochemical reaction that appears on a surface with an electrolyte solution presenting cathode and anode reactions. Similarly, steel structures protected by paint also rust because of aging or cracking of the paint or surface tension on corners. Bonding bolts with incomplete oil removal on bolt surfaces, bad coating angle, and inadequate coating easily result in rusting, and the situation gets worse if not noticed or mended in time.
Cioto et al. 5 demonstrated that delta ferrite affects the mechanical properties of any tensile resistance class, reducing its ability to limit micro failures at its weakest point (root radius). Micro failure occurs due to low capacity for the plastic deformation of a material’s surface (affected by phosphorus diffusion) during bolt assembly. This study found that bolt fatigue durability is affected by delta ferrite, which causes a reduction in the endurance limit independent of resistance class. Kumar and Gaur 6 display an overview of hydrogen embrittlement in fasteners. Hydrogen diffuses easily into the metal crystalline structure either as atoms or protons. Non-metallic inclusions such as sulfides and phosphorus favor hydrogen desorption, acting as catalysts. In general, there are two types of hydrogen embrittlement, the first being when environmental hydrogen assists failure through corrosion and the second from manufacturing processes. In most cases, hydrogen embrittlement causes fastener failures in high-hardness and high-strength fasteners that are electroplated.
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. When exposed to corrosive environments, hardened steel under tensile stress is likely to acquire brittle fractures called stress corrosion cracks. Flexibility changes with alloy components, yield strength, corrosive environments, tensile stress, and production residual stress.
In an analytical report on high-tensile bolt fractures with scanning electron microscope (SEM), 9 Ford found corrosion pits with intergranular fractures on the bolt surface. 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.
Based on the phosphorus brittle fracture mechanism, past research reports merely mention that phosphorus brittle fracture is caused by the zinc phosphate lubricant in the high-tensile bolt cold forming process that is used for reducing die damage. When the zinc phosphate coating is not removed before heat treatment, phosphorus (as phosphatizing) permeates the surface of the steel during the quenching heating process, resulting in brittle fracture.
Focusing on phosphorus brittle fracture, different process parameters were therefore established in this study to produce and test samples and further elucidate the effects of the phosphorus removal process on high-tensile bolts. We also performed various destructive and nondestructive tests to observe the relationships among phosphorus removal, heat treatment, and plating.
Basic theory
Heat treatment theory
Importance of heat treatment
Heat treatment is a method to change the physical properties of materials through heating, soaking, and cooling. 10 The metal material structure is mainly changed to acquire the expected mechanical or physical property improvement. The material is first heated to a given temperature, soaked for a certain period of time, and then certain properties of the material improved through transformation or diffusion precipitation and structure change at an appropriate cooling rate. The constant progress in human technology improves industrial product capabilities by employing strict manufacturing conditions. The research and development of materials such as superplastic alloys, memory alloys, and heat-resistant alloys allow them to be designed with specific functions conforming to user needs.
Heat treatment technology is therefore constantly improving along with the research and development of new materials. The mechanical properties of materials, including strength, hardness, and fatigue endurance, are improved through heat treatment, and various kinds of parts were not formerly used before reinforcement with heat treatment became available. General heat treatment contains annealing, normalizing, quenching, tempering, surface heat treatment, processing heat treatment, special heat treatment (martempering, austempering, and marquenching), and precipitation hardening, which are suitable for various alloys of steel, cast iron, aluminum, copper, and titanium.
Heat treatment of high-tensile bolts
For high-tensile bolts, the steel is first quenched to enhance its hardness and strength. This reduces its toughness, however, so high-temperature tempering is additionally done to further remove internal stresses within the quenched steel. This reduces its hardness and increases its toughness, allowing its structural and mechanical properties to be adjusted and improved.
Fracture of metal materials
Mechanical-force fracture
Mechanical-force fracture mainly results from applied stress, internal residual stress, or other stress and is divided into constant-force fracture and fatigue fracture. 11
Corrosive fracture
Corrosive fracture occurs on material surfaces and indirectly reduces material strength. 12 Located in subtropical island climates, the atmosphere of Taiwan has many corrosive factors affecting the strengths of materials, even resulting in fractures. Different types of corrosive fracturing present specific reaction mechanisms. For this reason, the type of corrosion and the reaction mechanism need to be identified before analyzing a corrosive fracture. Since corrosion is an electrochemical reaction, electrochemistry technology could be effectively utilized for testing and prevention.
High-temperature fracture
High-temperature fracture contains high-temperature chemical-force fracture and high-temperature mechanical-force fracture. High-temperature chemical-force fracture, also named high-temperature corrosive fracture, is metal surface oxidation and vulcanization with oxygen and sulfur in the atmosphere under high-temperature environments.
Requirements for high-tensile bolts
High-tensile bolts refer to fasteners with carbon steel and alloy steel bolt thread or similar thread and having tensile strength ranges between 1040 and 1370 MPa. These materials and their mechanical characteristics and manufacturing processes follow ISO898-1 specifications in order to ensure their reliability and avoid brittle fracturing. This is especially true for hydrogen-induced stress cracking corrosion requirements.
Research method
Bolts with carbon alloy steel 10B33 were selected for this experiment and divided into dephosphoring and non-dephosphoring groups. After heat treatment, bolts with and without dephosphoring first underwent a phosphorus removal test and analysis and bolts undergoing the electroplating process went through fracture analysis, tensile strength testing, hardness testing, metallographic structure observation, and destructive torque testing (Figure 1). The procedure and process classification are shown in Figure 2. Bolts without dephosphoring were divided into the hydrogen non-degassing (Sample A, without baking) and hydrogen degassing (Sample B, with baking) groups after heat treatment and electroplating. Dephosphored bolts were divided into hydrogen non-degassing (Sample C, without baking) and hydrogen degassing (Sample D, with baking) groups after heat treatment and electroplating. The experiments aimed to test and analyze preset process conditions and discuss the effects of a phosphorus removal process on high-tensile bolts.
Flow chart of the experiment.
Process classification for testing bolts with or without dephosphorus.
Sample preparation
DIN933 M8-1.25 mm × 50 mm, 10.9-level hexagonal head bolts were utilized as experimental samples and the 10B33 material that was tested. Its compositions are noted in Table 1. Bolt appearance is shown in Figure 3.
Compositions of the 10B33 material.
DIN933 M8-1.25 mm × 50 mm, 10.9-level hexagonal head bolts.
Dephosphoring process
Dephosphoring method
Dephosphoring processes for the screw industry in Taiwan are currently classified into acid and alkali. Acid removal focuses on hydrochloric acid, while alkali removal focuses on sodium hydroxide. An inhibitor is added in the cleaning process to prevent bolts from being over-eroded by the solvent. The dephosphoring process needs to be completed before heat treatment. The equipment is divided into (1) in combination with heat treatment based on an alkali cleaning solvent and (2) separate from heat treatment and based on an acid cleaning solvent. The second type (acid cleaner) was used in this study for the dephosphoring process. The chemical formula for the removal process is
Testing for phosphorus coatings after dephosphoring
Testing with a dephosphoring reagent determined whether a phosphate coating occurred when bolts were not fractured. The dephosphoring reagent reacts with and turns blue upon exposure to phosphorus. The higher the phosphorus content, the darker the blue reaction (Figure 4). This test is rapid and convenient.
Responses of the chemical reagent to phosphate coating: (a) acceptable (0.02) and (b) unacceptable (0.15).
Heat treatment conditions
An oil quench continuous heat treatment furnace was utilized to vary bolt heat treatments (Table 2).
Modification heat treatment conditions (sample material 10B33).
CP: carbon potential.
Testing methods
Material analysis
Wire used for general fastening production was mainly purchased from China Steel, except for special steel imported from Japan or the United States. The delivered wire rod arrived with material certificates attached, upon which the material’s composition and wire annealing furnace number were listed for reference to successive processes such as drawing, spheroidizing, forming, and heat treatment. Nevertheless, most companies outsource the wire purchased from China Steel for drawing and spheroidizing processes unless they have such equipment. In this case, mixture problems are likely to occur in those processes as a result of carelessness. To control the material and ensure the accuracy of successive experiments, a spectroscope was used for analyzing the wire material, mainly to verify chemical composition.
Fracture analysis
Different types of equipment and methods were utilized for analyzing fracture appearance and composition and deducing fracture mechanisms to discover the factors important in fracturing. Metallographic, tensile strength, destructive torque, and hardness tests were applied to the fracture analysis in this study. Observations of fracture appearance and fracture analysis were done with a low-power light microscope and a SEM.
Fracturing occurs at the weakest position of a metal structure, requiring the testing of several essential parameters related to the fracture process. Fracture morphology analysis can be used to study the basic issues of fracturing, namely, causes, properties, methods, mechanisms, toughness, process stressing, and crack expanding rate. Micro-component analysis, main body analysis, crystallography analysis, and fracture stress and strain analysis of a fracture section are required for in-depth research on the effects of a material’s metallurgical and environmental factors during fracturing.
Results and analysis
A metallographic structure analysis, dephosphoring test, fracture analysis, and mechanical properties analysis were done on all sample groups in this study, the results being summarized below.
Metallographic structure
Bolts formed without dephosphoring were coated with phosphate on the surface prior to experimental heat treatment. 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).
Metallography of bolts without dephosphoring being directly electroplated after heat treatment (200×).
Phosphatizing of bolts without dephosphoring being directly electroplated after heat treatment (200×).
No phosphate coatings or phosphatizing layers were found on sample surfaces after dephosphoring, mounting, and milling. Accordingly, 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.
Metallography of samples with dephosphoring (200×).
In sum, metallographic experiments were applied to observe structural changes during processes like electroplating after heat treatment of bolts with and without dephosphoring. In regard to metallographic structure changes after heat treatment, bolts with and without dephosphoring were indeed affected by the generation of phosphate coatings and phosphatizing layers.
Dephosphoring test
The reagent after testing bolts without dephosphoring was a dark blue, revealing a strong reaction of phosphorus (Figure 8(a)). Conversely, the lighter blue in Figure 8(b) means that the tested bolts were dephosphored.
Reactions of the reagent after testing bolts: (a) with or (b) without dephosphoring.
Fracture analysis
The macro comparison of fractures resulting from vertical tensile stress and transverse rotational stress in bolts without dephosphoring (Samples A and B) as well as bolts with dephosphoring (Samples C and D) are shown in Figures 9 and 10. These indicate that there were no explicit differences among Samples A to D under the two types of stress testing. Hence, based on macro fracture cross-sections, it is not effective for bolts to be dephosphored or not dephosphored. A comparison of micro fractures resulting from vertical tensile and transverse rotational stresses is shown in Figures 11 and 12. In regard to the cross-section of the fracture generated by vertical tensile stress, bolts without dephosphoring (Samples A and B) and bolts with dephosphoring (Samples C and D) presented similar vacancies and caves on fracture surfaces after heat treatment and electroplating. However, Samples A and B show grain boundary fracture vacancies caused by high temperatures while the fracture surfaces of Samples C and D merely appear as vacancies and caves resulting from ductile failure.
Macro image of fracture caused by vertical tensile stress: (a) to Sample A, (b) to Sample B, (c) to Sample C, and (d) to Sample D: (a) to Sample A, (b) to Sample B, (c) to Sample C, and (d) to Sample D.
Macro image of fracture caused by transverse rotation stress: (a) to Sample A, (b) to Sample B, (c) to Sample C, and (d) to Sample D.
Micro image of fracture caused by vertical tensile stress: (a) to Sample A, (b) to Sample B, (c) to Sample C, and (d) to Sample D.
Micro image of fracture caused by transverse rotation stress: (a) to Sample A, (b) to Sample B, (c) to Sample C, and (d) to Sample D.
Regarding the surfaces of fractures generated by transverse rotational stress, Samples A and B reveal cleavage fractures in a transgranular brittle fracture, Samples A and C present a hydrogen embrittlement crack, and Sample D merely appears to be ductile failure.
Analysis of mechanical properties
Various mechanical properties of bolt Samples A through D with and without dephosphoring after heat treatment and electroplating were tested (Table 3) and found to be acceptable. The experimental values were higher than standards, showing that dephosphoring has little or no obvious effect on the mechanical properties of bolts. This outcome applies only for immediate product use and does not consider effect over time.
Comparison of mechanical properties between bolts with and without dephosphoring after heat treatment.
Conclusion
We focused on 10.9-level bolts with and without dephosphoring that were directly electroplated or sand-blasted after heat treatment. These are further discussed with and without the degassing of hydrogen. Metallographic observation, fracture analysis, and mechanical property testing (tensile strength, yield strength, and destructive torque) were utilized for analyzing and discussing the effects of the dephosphoring process on high-intensity bolts and their correlations. Our results could be applied to the real production of high-intensity bolts and correspond to the requirements of the current automobile industry, specifically Volkswagen 7 and Ford, 8 that high-intensity bolts (except CL10.9 and CL12.90) need to have their surficial phosphate coating removed before heat treatment in order to avoid brittle fracturing. Our experimental results are summarized in the following conclusions:
Regarding the process, it is better to dephosphor bolts before heat treatment and use a dephosphoring reagent or metallographic observations for confirmation. Once the heat treatment of bolts is completed, phosphate coatings and phosphatizing layers are not easily removed.
In regard to mechanical properties (tensile strength, yield strength, and destructive torque), bolts with and without dephosphoring do not present significant differences; in other words, dephosphoring does not affect the mechanical properties of bolts.
Metallographic observations before and after heat treatment help to accurately judge the bolt dephosphoring process.
Footnotes
Acknowledgements
Buford Pruitt, Jr is appreciated for his editorial assistance.
Academic Editor: Stephen D Prior
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOST 104-2622-E-244-002-CC3.