1. Heat Treatment of Metals

2. Introduction and Hardening
This chapter discusses heat treatment
Controlled heating & cooling of metals to change characteristics
Hardening
Methods include direct hardening, surface hardening, and case hardening

3. FIGURE 2. 7. 1 Heat-treating temperatures for some alloy steels
FIGURE Heat-treating temperatures for some alloy steels. From Machinery’s Handbook 28, Industrial Press, New York, 2008

4. Figure 2.7.3 Flame hardening passes a torch over the surface
Figure Surface hardening creates a hard outer surface while keeping the core softer and tougher.
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Figure Flame hardening passes a torch over the surface
of the steel. Water is applied after the torch to quench the steel.
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5. Figure 2. 7. 4 Induction hardening of gear teeth
Figure Induction hardening of gear teeth. The glowing element heats teeth one at a time with electrical current. Note the water spray that quenches the teeth after hardening.
Courtesy of American Metal Treating Co.

6. FIGURE 2.7.5 Rolling, dipping, and pack methods for carburizing
low-carbon steels for case hardening.
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7. Methods of Heat Treatment
Tempering
Reduces brittleness
Annealing
Returns metals to pre-hardened condition
Normalizing
Before heat treatment improves results
Heat treatment of nonferrous metals
Heat treatments for aluminum alloys

8. Heat-Treating Furnaces and Safety
Furnaces heat metal to required temperature
Safety:
Use proper PPE to prevent severe burns
Gas furnaces: avoid dangerous build-up
Assume metal is too hot to touch
Completely submerge hot steel in quenching fluid to prevent fires

9. FIGURE 2. 7. 6 A dual-chamber box furnace
FIGURE A dual-chamber box furnace. The top unit can reach temperatures of 2200°F and is used for hardening. The lower chamber can reach 1200°F and is used for tempering.
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10. FIGURE 2.7.8 A pit-type furnace.
FIGURE This conveyor-type furnace is used to heat treat springs. The springs are loaded on one end of the furnace and exit at the other end after being heat treated.
Reprinted with permission from The Grieve Corporation. All rights reserved.
FIGURE A pit-type furnace.
Reprinted with permission from The Grieve Corporation. All rights reserved.

11. FIGURE An atmospheric furnace can preheat, heat, and quench steel without any exposure to air to minimize oxidation.
Lucifer Furnaces, Inc.
FIGURE An elevator furnace. The bottom lowers for loading and unloading of parts.
Lucifer Furnaces, Inc.

12. FIGURE In addition to standard PPE such as safety glasses, wear a heat-resistant face shield, long-sleeved fire-resistant jacket, and fire-resistant gloves when performing heat-treating operations, and always use tongs to handle hot metal.
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13. Hardness Scales and Testing
Methods for measuring & identifying hardness of steels after heat treatment
Two common systems for measuring: Rockwell hardness scale & Brinell hardness scale
Both calculate hardness from depth of indentation
FIGURE The basic principle of the Rockwell hardness test.
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14. A B
FIGURE (A) Some of the Rockwell hardness scales showing the penetrator and major load used, and the materials those scales are used to measure. (B) The Brale diamond and the 1/16 ball indenter.
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15. FIGURE 2.7.14 The major parts of an open loop or deadweight Rockwell hardness tester.
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FIGURE A closed loop tester uses a microprocessor-controlled motor to force the penetrator into the sample. Wilson Hardness Rockwell 2000 closed loop tester.
Courtesy of Wilson Hardness.

16. FIGURE 2. 7. 17 Installing an anvil and penetrator
FIGURE Installing an anvil and penetrator. Here the Brale point will be used for a C scale measurement.
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FIGURE Be sure the crank handle is in the forward position before beginning the hardness test.
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17. FIGURE 2.7.18 Check for or install proper weights on the B
FIGURE Check for or install proper weights on the
rear of the tester. All three weights equal the 150-kg major load for a C scale measurement.
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FIGURE (A) Adjusting the elevating screw to set the minor load. (B) Set the small needle near the dot, the large needle at vertical, and adjust the dial face so the “0” aligns with the large needle.
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18. FIGURE 2.7.20 Press the lever to trip the crank and apply the major load.
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FIGURE Record the hardness reading after releasing the major load. The result of this test is 50 on the C scale. This can be written as Rockwell C50, RC50, or HRC 50.
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19. FIGURE The principle of the Brinell hardness scale and formula for determining the HB number. “D” is the ball diameter, “d” is the impression diameter, and “F” is the weight of the load applied.
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FIGURE Brinell Hardness Number (BHN) ranges and the load sizes used to test those ranges.
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20. FIGURE 2. 7. 24 A Brinell microscope
FIGURE A Brinell microscope. The microscope is placed over the impression made by the test and the scale in the viewfinder is used to measure the diameter of the impression.
Courtesy of PTC Instruments