Tempering of martensitic steel for fasteners

Abstract

The research presented in this thesis aims to deepen our understanding of the effect of micro-alloying on the microstructure and mechanical property evolution during tempering of martensitic steel for fasteners. The ongoing trend of engine down-sizing has led to the need for stronger and more temperature resistant fasteners than currently available according to international standards. A new martensitic fastener steel called KNDS4 has been developed, that combines higher strength with improved resistance to hydrogen embrittlement. The higher strength is the result of the addition of small amounts of alloying elements such as Ti, V and Mo that can form alloy carbides. The improved resistance to hydrogen embrittlement is ascribed to the presence of nano-sized alloy carbides. However, in addition to enhancing strength and resistance to hydrogen embrittlement, alloy carbides are also able to improve the creep resistance of steels via pinning of dislocations at elevated temperature. It might therefore be possible to use the new fastener steel at higher service temperatures than the current high-strength fasteners. In order to optimize the properties of the new fastener steel, a fundamental understanding is needed of the relationship between the evolution of the microstructure and the hardness/strength during heat treatment. The research questions of the research are: (i) To which extent do different hardening mechanisms contribute to the strength of martensitic fastener steels? (ii) How do the different hardening mechanisms evolve as a function of time during annealing? (iii) What are the effects of a strong carbide forming element such as titanium on the microstructure and hardening mechanisms of martensitic fastener steel? (iv) Can the strength and temperature resistance of a martensitic fastener steel be improved by addition of carbide forming elements? The initial part of the research is based on a model alloy without the presence of any carbide forming elements that is compared to a model alloy with only one carbide forming element, Ti, in order to study the influence of a single alloy carbide on the microstructure and properties. Thereafter, industrial fastener steels, with multiple carbide forming elements, are studied, where complex alloy carbides form. Chapter 2 describes the industrial context that lead to the research described in this PhD-thesis. This background chapter consist of three main sections. Section 2.1 gives an introduction on bolted joints in the automotive industry. The bolted joint is defined as the system consisting of the bolt itself, and the components that are held together by the bolt. The purpose of the bolt, to keep the assembled components together by compression via a clamp force, is discussed and the distribution of external forces into the bolted joint is reviewed for different load cases. The influence of settlement (loss of clamping force due to localized plastic deformation in the joint after assembly) is reviewed and the fatigue properties of the bolt in a bolted joint are briefly discussed, in terms of distribution of an alternating external load into the bolted joint. The chapter ends with a review of the key mechanical properties of engine fasteners and an explanation of how current fastener steels are chosen. Section 2.2 introduces the concept of strength and hardness and temperature resistance of metals. The theory of dislocations and dislocation movement is briefly explained and the mechanisms behind different types of dislocation movements are presented. The link between dislocation movement and strength of metals is explained and the different microstructure components that contribute to the total strength and hardness of a metal are reviewed. Equations for quantification of the strengthening mechanisms are presented and literature values related to steel are given. The (nearly) instantaneous and time-dependent (creep) effects of an external load at elevated temperature on the strength and deformation of steel are described. Creep is the time-dependent and permanent deformation of a metal under constant external loading. The mechanical behavior during the different stages of creep is discussed and the microstructural mechanisms behind creep are reviewed. Expressions for different mechanisms of creep are given. Section 2.2 ends with a review of different methods to measure creep, with special focus on the indentation creep method, since this method was used for the research of this Ph.D. study. Section 2.3 presents martensite formation and precipitation-strengthening of martensite. The Fe-C phase diagram and the theory of martensite formation, according to the Bain theory, is discussed. The crystallography of lath martensite is presented and related to the strength via the hardening mechanisms presented in section 2.2. The evolution of the microstructure which takes place during tempering of martensite is summarized. Nucleation of precipitates, according to the classical nucleation theory, is presented and the energy terms which influence the nucleation of new phases are reviewed. The mechanisms of diffusion- and interface-controlled growth are presented. The Zener or diffusional growth is reviewed in more detail. Section 2.3 ends with a summary of the characteristics of TiC precipitates and the kinetic data related to the nucleation and growth of TiC in steel, since this precipitate type is specifically studied in the thesis. Chapter 3 is an experimental study of the evolution of the hardness and microstructure in Fe-C-Mn martensite (that is free of alloy carbides) during isothermal annealing at 300°C. The hardness near martensite block boundaries is significantly higher than the hardness inside the block matrix, due to a higher dislocation density in the regions adjacent to the block boundaries (called boundary regions). The boundary regions soften with increasing tempering time, whereas the nano-hardness of the tempered matrix remains approximately constant with increasing tempering time. The softening kinetics of Fe-C-Mn martensite can be described by three stages, which are related to the evolution of the microstructure: Stage I (0-5 min) is characterized by fast macroscopic softening kinetics that is strongly related to: (a) fast and simultaneous softening and reduction in area fraction of boundaries regions (b) fast reduction in area fraction of non-tempered matrix regions. Stage II (5-10 min) is characterized by slow macroscopic softening kinetics that is related to slow softening and reduction in area fraction of the boundaries regions. Stage III (10-60 min) is characterized by very slow softening kinetics that is related to very slow softening and reduction in area fraction of boundary regions. Chapter 4 is an experimental study of the influence of the addition of 0.042wt% Ti on the evolution of the microstructure and hardness of Fe-C-Mn steel during isothermal annealing at 300°C and at 550°C. The macroscopic hardness of Ti-containing and Ti-free Fe-C-Mn steel reduces rapidly during the first 5 minutes of tempering, due to (i) the redistribution of interstitially dissolved carbon into cementite and (ii) rapid recovery. The macroscopic hardness thereafter remains stable during continued annealing for the Ti-free steel, but the Ti-containing steel increases in hardness after 30 minutes of annealing at 550°C. The hardness increase of Ti-containing Fe-C-Mn-Ti steel is related to the formation of TiC-precipitates at 550°C. Nucleation of TiC-precipitates starts in the regions close to the martensite block boundaries (between 5-10 minutes) and subsequently nucleates in the block matrix (between 10-30 minutes) due to the higher dislocation density in the regions close to the block boundaries. The formation of TiC-precipitates slows down the recovery in the regions close to the martensite block boundaries, especially between 5 and 10 minutes of annealing. The growth of TiC-precipitates in martensite is simulated in good agreement with experimental observations with a model that takes capillarity effects, the overlap of the titanium diffusion fields and the effects of pipe diffusion of titanium atoms into account. Chapter 5 is a computational study on the evolution of the hardening mechanisms in Fe-C-Mn-Ti steel during isothermal annealing. The hardness of martensite is simulated as a linear addition of multiple strengthening mechanisms. This hardness model is combined with a microstructural model based on the Kampmann-Wagner-Numerical (KWN) approach for a multi-component and multi-phase system to simulate the nucleation and growth of TiC-precipitates. The model is fitted to experimental results and used to simulate the hardness contribution of different microstructure components as a function of annealing time. The two microstructural components which contribute most to the overall hardness of the investigated Fe-C-Mn-Ti steel are Fe3C precipitates (88 HV) and dislocations (54 HV) on a total of 284 HV. Both contributions decrease rapidly during the initial stages of annealing and stabilise after 10 minutes of annealing. The addition of titanium to the steel gives a minor hardness contribution via Ti-atoms in solid solution and TiC precipitates. Ti atoms in solid solution give a hardness contribution which increases slightly during the first few minutes of annealing and then remains stable (at 25 HV). The direct contribution of TiC precipitates to the overall hardness is limited (3.5 HV). However, TiC-precipitates also contribute to the overall hardness by pinning of dislocations during the recovery that takes place during the tempering. The model predicts that only a small volume fraction of TiC-precipitates forms during isothermal annealing at 550°C due to the large misfit strain (1.34 GJ/m3) and the low density of potential nucleation sites. Chapter 6 presents a comparative study of the evolution of mechanical properties at elevated temperature and the underlying microstructural mechanisms of ultra-high-strength and conventional high-strength steels for fasteners. The mechanical properties of the ultrahigh-strength steel KNDS4 of fastener grade 14.9 (strength 1400 MPa, yield-to-strength ration 0.9) and of conventional, high-strength steels 34Cr4 of fastener grade 12.9 (strength 1200 MPa, yield-to-tensile-strength ration 0.9) and 33B2 of grade 10.9 (strength 1000 MPa, yield-to-tensile-tensile-strength ration 0.9) are measured at room temperature and at elevated temperature. The alloy carbides in the steels are examined in order to investigate the underlying microstructural mechanisms that give rise to the different properties of the three fastener steels. KNDS4 steel has a higher yield strength ratio than both conventional high strength steels at 500°C, which have similar yield strength ratios at 500°C. Increasing the soaking time from 5 seconds up to 100 hours at elevated temperatures does not have an impact on the yield strength ratio. The nano-indentation creep rate shows a weak trend in which the tendency for deformation during constant load nano-indentation is lower in KNDS4 than in the 34Cr4 and 33B2 steels. This is measured both at similar indent depths and at the same indent time. The improved mechanical properties of the KNDS4 steel compared to the conventional high-strength steels are related alloy carbides in the microstructure that hinder dislocation movement. The alloy carbides in KNDS4 are smaller than the alloy carbides in 34Cr4 steel, and the properties are therefore better. Changing the standard industrial heat-treatment from an austenitization temperature of 940 to 1350°C can increase the hardness of KNDS4 by 8%. The increase stems from more effective dissolution of mainly Ti during the austenitization treatment. Titanium in solid solution enables nucleation and growth of precipitates, which generates precipitation strengthening during subsequent tempering. However, the standard industrial heat treatment results in a smaller martensite block size, which might be more beneficial for the toughness of the steel. The study of martensitic model alloys showed that the martensite block structure remain stable at temperatures up to 550°C, whereas the redistribution of alloying elements such as carbon is rapid and cannot be prevented. The study of the model alloys furthermore confirmed that addition of a strong carbide forming element, such as Ti, results in nucleation of a fine dispersion of alloy carbides that prevents recovery and thereby adds both precipitation strengthening and dislocation strengthening to the steel. Our study of the industrial fastener steels thereafter confirmed that alloy carbides in martensite increases the temperature resistance of the steel, by maintaining a high yield strength at elevated temperatures. The study of the industrial steels furthermore showed that the tendency to material creep at room temperatures is reduced in steels with alloy carbide precipitates. Development of more temperature resistant high strength steels for fasteners shall therefore be based on the strengthening mechanisms of grain boundaries and on alloy carbide precipitates. Our research furthermore showed that there is a need for further studies of traditional, axial creep testing, to fully understand and evaluate the beneficial effects of alloy carbides in martensitic steels. For the application of the existing KNDS4 steel we find that, independent of the heat treatment, the mechanical performance of KNDS4 fasteners at elevated temperature and the low nano-indentation creep rates are two strong indicators that fasteners made from KNDS4 steel might be used at higher service temperatures than traditional high strength fasteners, due to the presence of small alloy carbides in the microstructure of KNDS4. Higher strength of a fastener steel enables development of smaller, but stronger fasteners. These fasteners can be used in critical applications inside the engine, to down-size e.g. connecting rods, which will make it possible to significantly reduce the size and weight of modern combustion engines. Furthermore, the improved temperature resistance of new martensitic fastener steels will allow using the fastener at elevated service temperatures. These fasteners can therefore be used in applications where the temperature exceeds the recommended service temperature of 150°C (with the maximum upper boundary of 300°C) as stated in ISO898-1. This make is possible to reduce the use of highly alloyed high temperature fasteners (which are designed for service temperatures of 500°C or more) that are used in engines today due to the lack of cost efficient, resource-efficient, micro-alloyed fastener steels suitable for service at 300-500°C.