© COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON I, PART A rate of the metal.  The preheat temperature may vary from 150°F to 1000°F, but more commonly it is held in the 300°F to 400°F range.  The thicker the weld metal, the more likely will it be necessary to preheat, because the heat will be conducted away from the weld zone more rapidly as the mass increases. 1.5.2 Stress Relieving - Metals expand when heated and contract when cooled.  The amount of expansion is directly proportional to the amount of heat applied.  In a weldment, the metal closest to the weld is subjected to the highest temperature, and as the distance from the weld zone increases, the maximum temperature reached decreases.  This nonuni- form heating causes nonuniform expansion and contraction and can cause distortion and internal stresses within the weldment.  Depending on its composition and usage, the metal may not be able to resist these stresses and cracking or early failure of the part may occur. One way to minimize these stresses or to relieve them is by uniformly heating the structure after it has been welded.  The metal is heated to temperatures just below the point where a microstructure change would occur and then it is cooled at a slow rate. 1.5.3 Hardening - The hardness of steel may be increased by heating it to 50°F to 100°F above the temperature that a microstructure change occurs, and then placing the metal in a liquid solution that rapidly cools it.  This rapid cooling, known as "quenching," locks in place microstructures known as "martensite" that contribute to a metal's hardness characteristic.  The quenching solutions used in this process are rated according to the speed that they cool the metal, i.e.,  Oil (fast), Water (faster), Salt Brine (fastest). 1.5.4 Tempering - After a metal is quenches, it is then usually tempered.  Tempering is a process where the metal is reheated to somewhere below 1335°F, held at that tempera- ture for a length of time, and then cooled to room temperature.  Tempering reduces the brittleness that is characteristic in hardened steels, thereby producing a good balance between high strength and toughness.  The term toughness, as it applies to metals, usually refers to resistance to brittle fracture or notch toughness under certain environmental conditions.  More information on these properties will be covered later in this lesson and in subsequent lessons.  Steels that respond to this type of treatment are known as "quenched and tempered steels." 1.5.5 Annealing - A metal that is annealed is heated to a temperature 50° to 100° above where a microstructure change occurs, held at that temperature for a sufficient time for a uniform change to take place, and then cooled at a very slow rate, usually in a fur- nace.  The principal reason for annealing is to soften steel and create a uniform fine grain structure.  Welded parts are seldom annealed for the high temperatures would cause distortion.

Lesson 1
The Basics of Arc Welding
Lesson 2
Common Electric
Arc Welding Processes
Lesson 3
Covered Electrodes for Welding
Mild Steels
Lesson 4
Covered Electrodes for Welding Low Alloy Steels
Lesson 5
Welding Filler Metals for Stainless Steels
Lesson 6
Carbon & Low Alloy
Steel Filler Metals -
Lesson 7
Flux Cored Arc Electrodes Carbon Low Alloy Steels
Lesson 8
Hardsurfacing Electrodes
Lesson 9
Estimating & Comparing Weld Metal Costs
Lesson 10
Reliability of Welding Filler Metals


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