Chapter 17

Metallurgy

An old error is always more popular than a new truth.
—German Proverb

Introduction

The hardening and softening of metals using heat is essential to making the tools and products of the modern world—everything from kitchen knives to jet engine blades. This chapter focuses on steel because it is the most common metal that is heat-treated and the most used metal in a welding shop.

Metallurgy and heat-treatment is a complex subject. Metallurgists, physicists, and heat-treatment specialists often spend a lifetime in its study, so it is not possible to completely cover this subject in the space of these pages. For critical or complex applications, professional guidance and equipment are essential.

However, this chapter will cover several important heat-treating tasks that can be performed in a welding shop with limited equipment, including the heat treating of several types of tool steels. This chapter explains the advantages and limitations of the equipment needed to perform these processes, along with common heat-treatment problems and their solutions.

Because welding exposes metal to high temperatures, welders need to recognize the importance of metallurgy. Only by examining the crystalline nature of metals and how heat can cause changes in their crystal structure and properties can we understand how metals react to heat and the practical aspects of heating, quenching, tempering and annealing steel. In addition, this chapter explains the iron-carbon diagram, the steel classification systems, the effects of alloying elements, and compares the different types of cast iron. With this background we can then understand how to reduce the negative effects of welding heat on metals.

Section I – Iron-Based Metals

Iron & Iron Alloys

The element iron (Fe) is the most common element on the planet, making up much of the inner and outer core of the earth. Iron makes up all but a few percent of the contents of iron-based metals. These metals have very different properties depending on their carbon content.

Table 17-1 lists the major iron-based metals, their carbon content, characteristics and applications.

Table 17-1. Carbon content of major iron-containing metals.

Table 17-2 lists the main alloying elements found in the high-speed steels that are most often used in the manufacture of lathe and milling machine cutting tools and drills.

Table 17-2. Principal alloying elements of high-speed steels.

Section II – Why Steel Hardens

Structure of Metals

During the hot liquid state metals have no particular structure. There is no orderly, defined or regular organization among their atoms. However, at lower temperatures the atoms have less energy, move less rapidly, and atomic forces arrange them into particular structures or patterns called crystals. All metals and alloys are crystalline solids. Figure 17-1 shows the three most common crystal structures in metals and alloys.

Figure 17-1. The three most common crystal structures in metals and alloys.

Behavior of Pure Iron

As its temperature is gradually increased, the crystalline structure of iron changes, but still remains solid. Here are the crystal structures of pure iron at different temperatures:

From room temperature to 1670°F, it is body-centered cubic.

From 1670°F to 2535°F, it is face-centered cubic.

From 2535°F to its melting point of 2800°F, it is again body-centered cubic.

These changes between different crystalline structures for pure iron are not particularly important by themselves. However, if we add as little as one percent carbon by weight to iron, we make the iron-carbon alloy called steel. Because of the changes that occur in pure iron’s crystalline structure at different temperatures, an entirely new series of microstructures form. These microstructures not only have radically different physical properties from pure iron and from each other, their structures can be modified by applying heating and cooling cycles.

Iron-Carbon Diagram

The best way to visualize the interaction of iron and carbon and the resulting microstructures they form is to study the iron-carbon diagram. This diagram shows the microstructures that exist by adding a small percentage of carbon to iron at various temperatures.

Looking at this diagram in Figure 17-2, we can see that:

• Steel exists for iron-carbon alloys with between 0.2 and 2 percent carbon.
• Iron-carbon mixtures with more than 2% carbon are called cast iron.
• Depending on its carbon content and temperature, there are six different microstructures as represented by areas on the diagram:

-     Pearlite and ferrite

-     All pearlite

-     Pearlite and cementite

-     Austenite and ferrite

-     All austenite

• Austenite and cementite

• In the areas showing two microstructures simultaneously, the structure changes from all one microstructure, to a mixture of the two, to all the other microstructures as we move along the bottom axis, which represents the carbon content. For example, in the “pearlite and ferrite” area of the diagram (bottom left), the structure goes from all ferrite at 0.2% carbon, to a mixture of ferrite and pearlite, to all pearlite at 2% carbon. This means that in the areas of two microstructures, both the microstructure and its properties are altered with changes in its carbon content.
• Austenite exists only at a temperature of 1333ºF or higher and can contain up to 2% carbon.