Introduction

Because welding exposes metal to high temperatures, weldors need to have an understanding of how metals react to heat. To do this, we examine the properties of metals and look at how they are measured. Then we investigate the crystalline nature of metals and how heat can cause changes in their crystal structure and the metal's properties. We learn about the effects of alloying elements, study the iron-carbon diagram and explain the hardening, tempering, and heat treatment of steel. We also look at steel classification systems and consider the different types of cast iron. With this background, we then look at ways to reduce the negative effects of welding heat on metals.

Fundamentals

What is the difference between metallurgy and welding metallurgy?

Metallurgy is the overall field of extracting and applying metals. Welding metallurgy is a subdivision concerning the behavior of metals during welding, and the effects of welding on the metal's properties.

Why are weldors interested in metallurgy and welding metallurgy?

To solve routine fabrication and repair problems, weldors must understand:

• Effects of welding on the properties of metals.

• How to correct or minimize the negative effects of welding on metals.

• Processes of hardening, annealing and tempering metals, particularly steels.

Mechanical Properties of Metals

What are the four basic types of stresses?

See Figure 12-1.

• Tension

• Compression

• Shear

• Torsion

Figure 12-1: Four types of stress.

All stress, however...