582
STEEL.— Mechanical Properties.
tion of shearing and punching had been replaced byplaning and drilling.
“ 4. That annealing by itself or after tempering,negatives, like tempering, the alteration produced byshearing and punching.
“These various results are easily explained. Theshearing and punching machines produce in theneighbourhood of the parts subjected to their actiona local and extreme pressure. On the one hand, thelimit of elasticity of the metal is exceeded, and itmust fail to show the original extension beforerupture; but this does not explain the increaseobserved in the hardness and tenacity.
“ On the other hand, this pressure induces thesolution of the combined carbon (? Ed.), and pro-duces an actual tempering in the part acted uponby the shears and around the punch. These portionsthus acquire greater hardness, more tenacity, andare susceptible of only slight extension. The temperthus produced is more intense than that obtainedby sudden cooling. The pressure of the punchis sufficient to exceed the limit of resistance of themetal, and this effect can never be produced bytempering mild steel of small volume by means ofsimple cooling; in this latter case the pressure andthe effect it produces are considerably less. It isthus that a ring surrounding a drilled hole, andtempered by sudden cooling, gave a very differentresult than was obtained by rings surrounding holespunched in the same plate.”
The well-known and much-discussed effect of
TABLE XV
punching and shearing in reducing the strength ofsteel is thus clearly traced to an alteration in themetal within certain limits of the area of compression,whereby a portion of the steel is strained beyond itspower of endurance, and thus a weak point is setup from whence a fracture may the more easily bestarted. The same reasoning explains why a pieceof steel plate of medium hardness with the edge asit leaves the shears may be bent double withoutcracking, if it be doubled on itself in such a way thatthe face upon which the shear blade descended bebent inwards, while if it be bent outwards the platewill crack. In the first case the edge which hasbeen strained beyond its limit of extension is broughtinside the bend, and is therefore not subject tofurther extension, as it is if this portion is made toform the outside of the bend.
The experiments of Barba were made with themild steel of recent manufacture, produced by addingferro-manganese to the decarburized metal instead ofspiegeleisen; and the results were found to be verysimilar whether Bessemer or Siemens steel wereused. The chemical analysis of the metal undertreatment is not given ; but as the tensile strain wasabout 28 tons per inch, it is probable that the carbonwas not much above T5 per cent.
The celebrated Creuzot works publish the followingclassification of their steel, which will give a goodgeneral idea of the mechanical properties of steelof various tempers and qualities. — Engineering,1875, p. 119.
.—CLASSIFICATION OF CREUZOT STEEL,
A Class.
Number ofClassification.
Non-Tempered.
Tempered.
Load Corresponding to
Percentage of
Extension
Rupture.
Elastic Limit.
at Rupture.
Rupture.
Elastic Limit.
at Rupture.
Tons per
Tons per
Tous per
Tons per
Square Inch.
Square Inch.
Square Inch,
Square Inch.
1
48-31
24-72
13
74-16
45-64
*2
2
46-67
23-96
15
70-05
43-30
4-8
3
44-57
23-07
17
66-95
41-71
7-2
4
43-61
22-12
19
61-37
38-42
9-4
5
39-81
21-04
21
56-17
35-63
11-1
6
36-77
19-65
23
49-89
31-90
13-2
7
33-72
18-25
25
43-49
27-77
14-6
8
31-19
16-86
27
38-80
23-96
18-0
9
10
11
28-53
14-26
29
35-63
21-30
21-0
—
—
—
—
—
—
£ Class.
Number ofClassification.
1
2
3
4
5
6
7
89
10
11
Non-Tempered.
Tempered.
Load Corresponding to
Extension
Rupture.
Elastic Limit.
at Rupture.
Rupture.
Elastic Limit.
at Rupture.
Tons per
Tons per
Tons per
Square Inch.
Square Inch.
Square Inch.
Square Inch.
49-26
26-05
13
75-64
48-06
3-8
47-49
25-36
15
72-91
47-87
5-7
45-52
24-88
17
68-47
45-01
7-8
36-90
23-74
19
62-76
41-46
10-2
40-83
22-70
21
57-70
39-37
12-6
37-85
21-42
23
51-99
34-87
14-8
34-87
20-16
25
46-79
31-57
17-0
32-01
18-76
27
41-72
28-34
19-5
29-60
17-43
29
37-28
25-36
22-0
26-18
14-96
32
32-84
20-92
24-2
—
—
—