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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