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HEAT TREATMENT OF STEEL
By thermal treatment, steel may be made harder or softer, stresses induced or relieved, mechanical properties increased or decreased, crystalline structure changed, machinability enhanced, etc. The terms used to describe such heat treatments and their effects are listed below.
NORMALIZE
Normalizing consists of uniform heating to a temperature slightly above the point at which grain structure is affected (known as the critical temperature), followed by cooling in still air to room temperature. This produces a uniform structure and hardness throughout.
ANNEAL
When not preceded by a descriptive adjective, annealing consists of heating to and holding at a suitable temperature, then allowing to cool slowly. Annealing removes stresses, reduces hardness, increases ductility and produces a structure favorable for formability.
Full Anneal – This term is synonymous with annealing and is used to differentiate anneal from bright anneal, stress relief anneal, etc.
Spherodize Anneal – This treatment is similar to full annealing except the steel is held at an elevated temperature for a prolonged period of time, followed by slow cooling in order to produce a microstructure where carbides exist in a globular or spheroidal form.
Soft Anneal – When maximum softness and ductility are required without change in grain structure, steel should be ordered soft annealed. This process consists of heating to a temperature slightly below the critical temperature and cooling in still air.
Stress Relief Anneal – Stress relieving is intended to reduce the residual stresses imparted to the steel in the drawing operation. It generally consists of heating the steel to a suitable point below the critical temperature followed by slow cooling.
Bright Anneal – This process consists of annealing in a closely controlled furnace atmosphere which will permit the surface to remain relatively bright.
QUENCH
Quenching consists of heating steel above the critical range, then hardening by immersion in an agitated bath of oil, water, brine or caustic. Quenching increases tensile strength, yield point and hardness. It reduces ductility and impact resistance. By subsequent tempering some ductility and impact resistance may be restored, but at some sacrifice of tensile strength, yield point and hardness.
TEMPER
Tempering is the reheating of steel, after quenching, to the specified temperature below the critical range, then air cooling. It is done in furnaces, oil or salt baths, at temperatures varying from 300 to 1200°F. Low tempering temperatures give maximum hardness and wear resistance. Maximum toughness is achieved at the higher temperatures.
RELATIONSHIP OF HARDNESS TO TENSILE
STRENGTH OF CARBON & ALLOY STEEL
|
Brinell
Indentation
Diameter
mm
|
Brinell Hardness
Number |
Rockwell Hardness
Number |
Tensile
Strength
(Approx.
1000 psi)
|
|
Standard Ball
|
Tungsten
Carbide Ball
|
B Scale |
C Scale |
|
2.45
2.50
2.55
2.60
2.65
|
—
—
—
—
—
|
627
601
578
555
534
|
—
—
—
—
—
|
58.7
57.3
56.0
54.7
53.5
|
347
328
313
298
288
|
2.70
2.75
2.80
2.85
2.90 |
—
—
—
—
—
|
514
495
477
461
444
|
—
—
—
—
—
|
52.1
51.0
49.6
48.5
47.1
|
274
264
252
242
230
|
2.95
3.00
3.05
3.10
3.15 |
429
415
401
388
375
|
429
415
401
388
375
|
|
45.7
44.5
43.1
41.8
40.4
|
219
212
202
193
184
|
|
3.20
3.25
3.30
3.35
3.40
|
363
352
341
331
321
|
363
352
341
331
321
|
—
—
—
—
—
|
39.1
37.9
36.6
35.5
34.3
|
|
3.45
3.50
3.55
3.60
3.65
|
311
302
293
285
277
|
311
302
293
285
277
|
|
33.1
32.1
30.9
29.9
28.8
|
149
146
141
138
134
|
3.70
3.75
3.80
3.85
3.90 |
269
262
255
248
241
|
269
262
255
248
241
|
—
—
—
—
100.00
|
27.6
26.6
25.4
24.2
22.8
|
130
127
124
120
116
|
|
3.95
4.00
4.05
4.10
4.15
|
235
229
223
217
212
|
235
229
223
217
212
|
99.0
98.2
97.3
96.4
95.5
|
21.7
20.5
—
—
—
|
114
111
104
103
100
|
4.20
4.25
4.30
4.35
4.40 |
207
201
197
192
187
|
207
201
197
192
187
|
94.6
93.8
92.8
91.9
90.7
|
—
—
—
—
—
|
99
97
94
92
90
|
|
4.45
4.50
4.55
4.60
4.65
|
183
179
174
170
167
|
183
179
174
170
167
|
90.0
89.0
87.8
86.8
86.0
|
—
—
—
—
—
|
89
88
86
84
83
|
4.70
4.80
4.90
5.00
5.10 |
163
156
149
143
137
|
163
156
149
143
137
|
85.0
82.9
80.8
78.7
76.4
|
—
—
—
—
—
|
82
80
73
71
67
|
5.20
5.30
5.40
5.50
5.60
|
131
126
121
116
111
|
131
126
121
116
111
|
74.0
72.0
69.0
67.6
65.7
|
—
—
—
—
—
|
65
63
60
58
56
|
* This table, which is based on ASTM A 370-68, Table lll, lists the approximate relationship of hardness values to corresponding approximate tensile strength values of steels. Some compositions and processing histories may deviate from these relationships. The data in this table do not represent hardness-to-tensile strength conversions for austenitic, ferritic, and martensitic stainless steel. If more precise conversions are required, they should be developed for each specific composition and heat treatment. Related Rockwell superficial hardness numbers, if of interest, may be found in ASTM A 370-68.
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