Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel - By John D. Verhoeven (2005).pdf

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Metallurgy of Steel for Bladesmiths & Others
who Heat Treat and Forge Steel
John D. Verhoeven
Emeritus Professor
Iowa State University
March 2005
Preface
For the past 15 years or so I have been working with practicing bladesmiths on
problems related to forging and heat treating steel blades. It has become apparent to me
in that time that there is a need for a book that explains the metallurgy of steel for people
who heat treat and forge steels and have had no formal metallurgical education. This
book is an effort to provide such a treatment. I have discovered that bladesmiths are
generally very quick to catch on to the ideas of metallurgy and consequently an attempt
was made to set the level of detail presented here for the needs of those wanting a fairly
complete understanding of the subject.
Most chapters in the book contain a summary at the end. These summaries
provide a short review of the contents of each chapter. It may be useful to read these
summaries before and perhaps after reading the chapter contents.
The Materials Information Society, ASM International, has published a book that
contains a wealth of information on available steels and is extremely useful to those who
work and heat treat steel: Heat Treater's Guide, Practices and Procedures for Irons and
Stteels, 2nd Edition, (1995), Materials Park, OH 44073. A major goal of this book is to
provide the necessary background which will permit a practicing metal worker to
understand how to use the information in the ASM book, as well as other handbooks
published by ASM International.
I would like to acknowledge the help of two bladesmiths who have contributed to
this book in several ways, Alfred Pendray and Howard Clark. Both men have helped me
understand the level of work being done by U.S. bladesmiths and they have also
contributed to some of the experiments utilized in this book. They are also responsible
for the production of this book because of their encouragement to write it. In addition I
would like to acknowledge many useful discussions with William Dauksch and my
colleague, Prof. Brian Gleeson, who made many useful suggestions on the stainless steel
chapter.
I am particularly indebted to Iowa State University and their Materials Science
and Engineering Department for providing me with the opportunity to teach metallurgical
engineering students about steel for over two decades, as well as to the Ames Laboratory,
DOE, that supported most of my research activity. Many of the pictures and methods of
presentation in this book result from my experience teaching students and doing research
at Iowa State University.
My professional career has been supported by publicly funded institutions.
Therefore, I grant any user copyright permission to download and print a copy of this
book for personal use or any teacher to do the same for their students. I do not grant
rights to the text for commercial uses. The copyrights to all figures with citations belong
to the original publishers. Copyright permissions were obtained for inclusion of these
figures.
ii
Index Index
Chapter Title of chapter or subtopic
Page
1
Pure Iron
1
Summary
4
2
Solutions and Phase Diagrams
5
Solutions
5
Phase Diagrams
6
Summary
7
3
Steel and the Fe-C Phase Diagram
8
Low Carbon Steels (Hyoeutectoid Steels)
10
High Carbon Steels (Hypereutectoid Steels)
11
Eutectoid Steels
13
The A 1 , A e1, A c1 , A r1 Nomenclature
15
References
16
Summary
17
4
The Various Microstructures of Room Temperature Steel
19
Optical Microscope Images of Steel Grains
19
Room Temperature Microstructures of Hypo- and Hypereutectoid Steels
20
Microstructure of Quenched Steel
23
Martensite
24
Two Types of Martensite
25
The M s and M f Temperatures
26
Martensite and Retained Austenite
27
Bainite
29
Spheroidized Microstructures
32
Summary
34
5
Mechanical Properties
36
The Tensile Test
36
The Hardness Test
38
The Notched Impact Test
42
Fatigue Failure and Residual Stresses
45
References
47
Summary
48
6
The Low Alloy AISI Steels
50
Manganese in Steel
52
Effect of Alloying Elements on Fe-C Phase Diagram
54
References
56
Summary
56
7
Diffusion
58
Carburizing and Decarburizing
61
References
63
Summary
64
8
Control of Grain Size by Heat Treatment and Forging
66
Grain Size
66
Grain Growth
67
New Grains formed by Phase Transformation
69
New Grains formed by Recrystallization
70
Effects of Alloying Elements
72
Particle Drag
73
Solute Drag
74
References
76
Summary
76
8
Hardenability of Steel
78
IT Diagrams
79
Hardenability Demonstration Experiment
83
CT Diagrams
85
The Jominy End Quench
88
Hardenability Bands
91
References
91
Summary
92
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10
Tempering
95
Tempered Martensite Embrittlement (TME)
96
Effect of %C on toughness
97
Effect of Alloying Elements
98
References
101
Summary
101
11
Austenitization
103
Single Phase Austenitization
103
Homogenization
106
Austenite Grain Growth
106
Two-Phase Austenitization
108
References
110
Summary
110
12
Quenching
113
Special Quenching techniques
113
Martempering
114
Austempering
115
Variation on Conventional Austempering
117
Characterization of Quench Bath Cooling Perfomance
120
Oil Quenchants
122
Polymer Quenchants
124
Salt Bath Quenchants
124
References
125
Summary
126
13
Stainless Steels
128
Ferritic Stainless Steels
129
Martensitic Stainless Steels
132
Optimizing Martensitic Stainless Steels for Cutlery Applications
134
Example Heat Treatment using AEB-L
139
Austenitic Stainless Steels
141
Precipitation Hardening Stainless Steels (PHSS)
145
References
147
Summary
147
14
Tool Steels
151
Tool Steel Classification
151
The Carbides in Tool Steels
153
Special heat treatment effects with tool steels
155
References
157
Summary
158
15
Solidification
159
Factor 1 - Microsegregation
160
Factor 2 - Grain Size and Shape
165
Factor 3 - Porosity
166
References
168
Summary
168
16
Cast Irons
170
Gray and White Cast Irons
171
Ductile and Malleable Cast Iron
179
References
182
Summary
183
Index
184
Appendix A - Temperature Measurement
A1
Thermocouples
A1
Radiation Pyrometers
A3
References
A7
B – Stainless Steels for Knife-makers
B8
iv
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1 - Pure Iron
Most steels are over 95% iron, so a good starting point to understanding steel is to
study the nature of solid iron. Consider the following experiment. A 1 inch diameter bar
of pure iron is sectioned to form a thin disk in the shape of a quarter. A face of the disk is
now polished on polishing wheels starting first with a coarse grit polish and proceeding in
steps with ever finer grits until one ends up with the face having the appearance of a
shinny mirror. The shinny disk is now immersed for around 20-30 seconds in a mixture
of 2 to 5 % nitric acid (HNO 3 ) in methyl alcohol (called nital, nit for the acid and al for the
alcoho l), a process known as etching. The etch causes the shinny surface to become a dull
color. If the sample is now viewed in an optical microscope at a magnification of 100x,
it is found to have the appearance shown on the right of Fig. 1. The individual regions
such as those numbered 1 to 5 are called iron grains and the boundaries between them,
such as that between grains 4 and 5
highlighted with an arrow, are
called grain boundaries. The
average size of the grains is quite
small. At the 100x magnification of
this picture a length of 200 microns
is shown by the arrow so labeled.
The average grain diameter for this
sample has been measured to be
125 microns, where 1 micron =
0.001 mm. Although a small
number, this grain size is much
larger than most commercial irons.
( It is common to use the term µm for
micron and 25 µm = 0.001 inches = 1 mil.
The thickness of aluminum foil and the
diameter of a hair both run around 50 µm. )
The basic building blocks of solids like salt and ice are molecules, which are units
made up of two or more atoms. For example, sodium + chlorine in table salt (NaCl) and
hydrogen + oxygen in ice (H 2 O). In metals, however, the basic building blocks are the
individual atoms of the metal, i.e., iron (Fe) atoms in a bar of iron, or copper (Cu) atoms
in copper wire. Each one of the grains in Fig. 1.1 is what we call a crystal. In a crystal
made up of atoms, all of the atoms are uniformly arranged on layers. As shown in Fig.
1.2, if you draw lines connecting the centers of the atoms you generate a 3-dimensional
array of little cubes stacked together to fill space. In iron at room temperature the cubes
have an atom at each of the 8 corners and one atom right in the middle of the cube. This
crystal structure is called a BCC (body centered cubic) structure, and the geometric
arrangement of atoms is often called a BCC lattice . Notice that the crystal lattice can be
envisioned as 3 sets of intersecting planes of atoms with each plane set parallel to one
face of the cube. Iron with a BCC structure is called ferrite . Another name for ferrite is
alpha iron, or α -iron, where α is the symbol for the Greek letter a.
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