Steels: Microstructure and Properties

H.K.D.H. Bhadeshia, R.W.K. Honeycombe

• Publisher: Butterworth-Heinemann
• Year: 2017
• ISBN:
  9780081002728
• Edition: 4
• Topic: Technology & Engineering
• Subtopic: Mining Engineering
• Index: Technology & Engineering

Steels: Structure and Properties, Fourth Edition is an essential text and reference, providing indispensable foundational content for researchers, metallurgists, and engineers in industry and academia. The book provides inspiring content for undergraduates, yet has a depth that makes it useful to researchers.

Steels represent the most used metallic material, possessing a wide range of structures and properties. By examining the properties of steels in conjunction with structure, this book provides a valuable description of the development and behavior of these materials—the very foundation of their widespread use.

The new edition has been thoroughly updated, with expanded content and improved organization, yet it retains its clear writing style, extensive bibliographies, and real-life examples.

• Contains a new chapter on nanostructured steels, with new content integrated into an existing chapter to describe the physical metallurgy of coatings, surface treatments, and multivariate high-performance steels
• Includes derivations with important equations so that students from a broad range of subjects can appreciate the issues without being bogged down in mathematics
• Presents new micrographs and figures that reflect the resolution and capabilities of modern instruments

Chapter 1

Iron and Its Interstitial Solutions

Abstract

Pure iron is remarkable in its complexity, not only because of its many allotropic forms. There are hidden features related to magnetism which, for example, make the expansion coefficient of austenite greater than that of the more loosely packed ferrite – one consequence of this is that austenitic steels deteriorate when subjected to a combination of stress and thermal fluctuations. We discuss here the choreography of atoms during solid-state phase transformations together with the role and behaviour of interstitial atoms such as carbon, nitrogen and hydrogen. The mobility of substitutional solutes is described to emphasise some counterintuitive observations such as the fact that heavy atoms like molybdenum actually diffuse faster than the iron in which they are dissolved.

Keywords

Allotropes of iron; Magnetic properties; Interstices; Transformation mechanisms; Diffusion; Interstitial solution

1.1 Introduction

Iron is created in the stars, which at first fuel themselves through the fusion of hydrogen into helium. When hydrogen is exhausted, the fusion of helium leads to the creation of carbon. This exothermic process of nuclear burning can, in sufficiently massive stars, lead to the formation of even heavier elements, the sequence ending with the creation of iron. Iron is the most stable element in the universe and further fusion to form even heavier elements does not release energy. As a consequence, the fuel supply becomes exhausted and the star collapses into states that depend on its mass. If the core becomes so heavy that it cannot support its own gravity, a giant explosion (a supernova) occurs that produces in a short time scale, most of the elements beyond iron.

Impressive as the cosmological origins of iron and carbon are, the story that truly is worth telling can be witnessed in every aspect of terrestrial life. Such is the success of iron, that steel forms the ‘gold-standard’ against which emerging materials or supreme acts of endeavour are compared. What is often not realised is that this is a moving standard, with notoriously regular and exciting discoveries being made in the context of iron and its alloys. This is why steel remains the most successful and cost-effective of all materials, with more than a billion tonnes being consumed annually in improving the quality of life. This book attempts to explain why steels continue to take this pre-eminent position, and examines in detail the phenomena whose exploitation enables the desired properties to be achieved.

One reason for the overwhelming dominance of steels is the endless variety of microstructures and properties that can be generated by solid-state transformation and processing. Therefore, in studying steels, it is useful to consider the behaviour of pure iron first, then the iron-carbon alloys, and finally the many complexities that arise when further solutes are added.

Pure iron is not an easy material to produce. It has nevertheless been made with a total impurity content less than 60 parts per million (ppm), of which 10 ppm is accounted for by non-metallic impurities such as carbon, oxygen, sulphur and phosphorus, with the remainder representing metallic impurities. Iron of this purity can be extremely weak when reasonably sized samples are tested: the resolved shear stress needed to induce slip in single crystal at room temperature can be as low as 10 MPa, while the yield stress of a polycrystalline sample at the same temperature can be well below 50 MPa. However, the shear strength of small single c...