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HARDNESS OF METALS
hardness
Hardness and cutting speeds
The purpose of this page is to determine appropriate cutting speeds mainly for amateur lathe users. The cutting speed tables are most dependent on the hardness of the material and the properties of cutting tools. The tools can be simply divided into two groups:
High speed steel HSS/tool steel/carbon steel group or the carbide group of tools with solid carbide, brazed carbide or replaceable carbide tips (often referred to as indexed tips if they can be rotated or flipped over to use multiple cutting edges).
Equations which allow rough estimation of Cutting speed (Cs) from the material hardness (Brinell hardness number, Bhn) were described earlier:
Cs = 18,000 / Bhn
... ft/min with HSS
Cs = 5,500 / Bhn
... m/min with HSS
Cs = 68,000 / Bhn
... ft/min carbide
Cs = 20,600 / Bhn
... m/min carbide
Measurement of Hardness
Some tables have been included to show the hardness of various metals. There are several methods for measuring hardness and they can be divided into three groups.
The first depends in creating an indentaion on the surface by applying a specific force on an object such as a hardened ball. The extend of indentation is used to measure hardness. One of the oldest method is the Brinell hardness number (Bhn) which is typically measured by applying 3000 Kg force to a 10 mm diameter hardened ball and measuring the diameter of the dent produced. There are other indentation methods including Rockwell hardness which uses a cone shaped diamond indentation device but there are three subgroups labelled Rockwell A, B, and C.
The second group measures elastic rebound. The simplest of these involve dropping a hardened steel ball onto the polished surface of the material to be tested. When the ball bounces back up, the height of rebound is a measure of hardness, which is related to the elasticity of the material.
The third group developed in 1812 measures hardness by determining whether one material is hard enough to scratch another material in the series. The hardest material on the Mohs scale is diamond with a hardness rating of 10, and the softest is talc. Number 9 is an oxide of aluminium which forms sapphire and ruby and also carborundum used in aluminium oxide grinding wheels. Masonary drills are 8.5 and knives 5.5 on this scale.
Hardness of materials
Although ideal cutting speed may be affected by feed rates and depth of cut, these are not as important as the hardness of the material being cut. That is why I looked for tables that give us some way to correlate cutting speed and hardness. Then this leads us to attempting to understand the effect of hardening and tempering of carbon steel on hardness.
Although the color change is no longer considered an accurrate method of tempering steel to the required degree of hardness, it is still used by small workshops and amateurs and I think it worth presenting here.
Rather than merging tables I have kept them separate in most cases so that I can give appropriate attribution to the original web sites where they were found. These links will be useful to people who would like to know more about the fascinating but complex topic of metalurgy.
Approximate comparison of scales of hardness
Brinell (Bhn)
Vickers
Rockwell B (HRb)
Rockwell C (HRc)
120
120
67
—
150
150
88
—
200
200
92
—
240
250
100
22
300
320
—
32
350
370
—
38
400
420
—
43
450
475
—
47
500
530
—
51
600
680
—
59
650
750
—
62
A comparison of the various scales of hardness measurement, for the convenience of conversion from one scale to another. The values are reasonable for most metals but conversion errors can occur if the material is prone to work hardening.
CREDITS
From Table 3.1 in a book 'Tribology Handbook' (Second Edition), 1995
Downloaded from the following site on 18 February 2022, modified and recoded.
https://www.sciencedirect.com/topics/engineering/brinell-hardness
Recommended Cutting Speeds for Turning Ferrous Metals in a lathe
Based on Brinell hardness number including the effects of heat treatment.
Feed rate 0.012 inch or 0.3 mm per revolution
and depth of cut 0.125 inches or 3.175 mm.
Material
Material
Condition
Hardness
Bhn
Cutting Speed
ft/min
Cutting Speed
m/min
High
Speed
Steel
Carbide
High
Speed
Steel
Carbide
Plain Carbon Steels
Free Machining (Resulphurized)
AISI
B1111,B1112,B1113,
1113, 1119, 1212, 1213
HR, A
CD
100-150
150-200
160
180
500
600
48
55
152
183
AISI
1108, 1115, 1118, 1120, 1126
HR, A
CD
100-150
150-200
140
150
450
500
43
46
137
152
AISI
1132, 1137, 1140, 1145, 1151
HR, A, N, CD
Q & T
Q & T
Q & T
175-225
275-325
325-375
375-425
130
90
50
30
500
250
175
140
40
27
15
9
152
76
53
43
Plain Carbon Steels
AISI
1012, 1015, 1018, 1019,
1020, 1022, 1024, 1025
HR, A, N, CD
HR, A, N, CD
HR, A, N, CD
CD
100-125
125-175
175-225
225-275
140
120
100
70
500
400
350
300
43
37
30
21
152
122
107
91
AISI
1027, 1029, 1030, 1032,
1035, 1037, 1040, 1043,
1045, 1047, 1050
HR, A, N, CD
HR, A, N, CD
Q & T, N, CD
Q & T, N
Q & T
Q & T
125-175
175-225
225-275
275-325
325-375
375-425
120
100
70
60
50
40
400
350
300
240
200
175
37
30
21
18
15
12
122
107
91
73
61
53
AISI
1055, 1060, 1065, 1070, 1074,
1080, 1085, 1090, 1095
HR, A, N, CD
HR, A, N, CD
Q & T, N, CD
Q & T, N
Q & T
Q & T
125-175
175-225
225-275
275-325
325-375
375-425
100
90
65
55
45
30
375
325
275
225
180
150
30
90
27
17
14
9
114
99
84
69
55
46
Free Machining Alloy Steels
(Resulphurized)
AISI
3140, 4140, 4150, 8640
HR, A, N, CD
HR, A, N, CD
Q & T
Q & T
Q & T
175-200
200-250
250-300
300-375
375-425
125
100
70
60
40
450
400
325
225
150
38
30
21
18
12
137
122
99
69
46
Alloy Steels
AISI
1320, 2317, 2512, 2517, 3115,
3120, 3125, 3310, 3316, 4012,
4017, 4023, 4028, 4320, 4615,
4620, 4720, 4815, 4820, 5015,
5020, 5024, 5120, 6118, 6120,
6317, 6325, 6415, 8115, 8615,
8620, 8625, 8720, 8822, 9310,
9315
HR, A, CD
HR, A, N, CD
Q & T, N, CD
Q & T, N
Q & T, N
Q & T
150-175
175-220
220-275
275-325
325-375
375-425
110
80
70
60
50
40
400
350
300
250
200
175
34
24
21
18
15
12
122
107
91
76
61
53
KEY
Bhn
Brinell hardness number
A
Annealed
AC
As Cast
CD
Cold Drawn or Cold Rolled
HR
Hot Rolled
N
Normalized
Q & T
Quenched (Hardened) and Tempered
ST & A
Solution Treated and aged
Feed Rate
Based on 0.012 inch per revolution
Depth of Cut
Based on 0.125 inch
Downloaded 18 February 2022, Modified and recoded.
Used under the Creative Commons license
from Wisc-Online created by Wisconsin’s Technical Colleges.
https://www.wisc-online.com/LearningContent/mtl11802/MLT11802.htm
Typical Brinell Hardness Numbers for Metals
Material
Brinell Hardness Number (Bhn)
lead
5
Aluminium (pure)
15
Copper
35
Soft brass
60
Aluminium (hardened)
75
Mild steel
130
Annealed chissel steel
235
White cast iron
415
Nitrided surface
750
Glass
1550
Rhenium diboride
4600
CREDITS
Downloaded 18 February 2022, Modified and recoded.
https://www.engineeringtoolbox.com/bhn-brinell-hardness-number-d_1365.html
Brinell Hardness of Soft Bearing Materials
Material
Brinell hardness
Tin base Babbitt
20–30
Lead base babbitt
15–20
Alkali-hardened lead
22–26
Cadmium base
30–40
Copper lead
20–30
Tin bronze
60–80
Lead bronze
40–70
Aluminium alloy
45–50
Silver plus overlay
25
CREDITS
From section 2.9.3 'Bearing materials' of
Applied mechanics by J. Carvill,
in Mechanical Engineer's Data Handbook, 1993
https://www.sciencedirect.com/topics/engineering/brinell-hardness
Downloaded modified and recoded on 18 February 2022.
Tempering temperatures and hardness for some carbon steel tools
Tool
Tempering Temperature
o
C
Hardness
(Rockwell or Brinell)
Milling cutters, Taps
150-220
o
C
HRc 60-64
Dies, punches for swaging or cold forging
220-260
o
C
HRc 55-99
Tools subjected to shock e.g. chisels
280-300
o
C
HRc 53-56
Hammer dies
350-450
o
C
Bhn 300-444
CREDITS
This table was extracted from an excellent article on hardening and tempering at the link below. It is recommended to those who want to know more about the effects of carbon content, temperature and time required for tempering steel that has been quenched. The table was downloaded on 18 February 2022, modified and recoded from an original
under creative commons license. The original article can be found in
Tempering of Steel: Stages and Classification | Heat Treatment | Metallurgy
which can be found at the following link:
https://www.engineeringenotes.com/metallurgy/steel/tempering-of-steel-stages-and-classification-heat-treatment-metallurgy/26205
Hardening Temperatures
When carbon steel is hardened, the temperature of the steel can be estimated from its incandescent glow when it is heated to a high temperatures before quenching in oil or water. This table has been compiled to indicate approximate colors observed in the glowing steel.
Temperature
Color of Heated Steel
o
F
o
C
2552
1400
White welding temperature
2372
1300
Yellow-White
2192
1200
Orange-Yellow
2012
1100
Orange-red
1832
1000
Bright Cherry Red
1652
900
Cherry Red
1472
800
Dull Cherry Red
1077
581
Red heat visible in sunlight
975
525
Red heat visible in daylight
CREDITS
Colors selected by Evan Lewis from a table in
The 'Engineers Black Book' by Pat Raff
p54, 3rd edition 2018
Quenching and Tempering (Q & T)
Carbon tool steel can be quench-hardened which is usually done by heating to a bright red hot temperature and quenched by plunging it into water or oil, or some other cooling agent. With heat the steel forms a structure which has carbon atoms dissolved in a super-saturated state. As it cools various crystal structures form, incorporating iron (ferrite) and carbon. The structures depend on the rate of cooling. When cooled very rapidly (within seconds) a cubic crystal structure is locked in, creating Martensitic steel. This structure is extremely hard, but also very brittle. As a result, the quenched material is prone to chipping and breaking easily, and may crack during rapid cooling.
Quenching is then followed by tempering to allow the structure to relax a bit. The temperature used for tempering varies depending on the hardness and strength required for different applications. Higher temperatures result in bigger changes in the structure resulting in softer but stronger metal. Typical tempering temperatures are shown in this table.
The temperature has traditionally been judged by the
color
of the steel which is also shown in this table. It is not absolutely precise but very convenient. An area of the steel is polished. When it is gradually heated the surface oxidizes and changes through the colors of the rainbow, determined by the thickness of the oxidized layer which causes interference with light of certain wavelengths. (The color produced has a wavelength which is twice the thickness of the oxide layer.) This color is only maintained for 2-3 minutes as the thickness of the layer increases with time.
If the tip of a tool, such as a wood chisel, needs to be tempered it can be heated from the non-cutting end, and as the heat conducts down the length of the tool, the tip eventually gets hot enough to turn the correct shade of yellow shown in the table. Heating can then be stopped by plunging in oil, locking in the appropriate crystal structure. Colors of the rainbow can be seen along the length of the tool with the body of the tool softer, but stronger than the cutting edge. Alternatively the metal can be heated to the exact temperature in a controlled oven and then cooled in air to give uniform hardness.
Temperature
Color of the oxide layer on heated carbon steel
o
F
o
C
used for tempering tool steel
600
316
Scrapers, spokeshaves
560
293
Screwdrivers, springs, gears
540
282
Cold chisels, center punches
520
271
Small Taps <= 1/4 inch (6mm)
500
260
Axes, wood chisels, drifts, taps>1/4 inch, nut taps, thread dies, press tools
480
249
Twist drills, large taps, knurls
460
238
Dies, punches, bits, reamers
450
232
Twist drills for hard use
440
227
Lathe & milling tools, reamers, scrapers
430
221
Edge tools, reamers
420
216
Knives, hammers
CREDITS
This table was downloaded on 18 February 2022, modified and recoded from an original
under creative commons license. The original article can be found in
Engineering ToolBox, (2009). Steel Tempering Colors
at the following link online:
https://www.engineeringtoolbox.com/tempering-colors-steel-d_1530.html
Ride The Gear Train
Gear train calculations for engineer's lathes.
A free online program to calculate gear trains for any screw thread with the gear wheels that you own.
• Metric or imperial threads on
• Metric or imperial lathes
• Metric/imperial conversion gears.
• Feed rates, worm gears.
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• Extensive help file, and video tutorials (below).
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•
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RECENT CHANGES
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QUICK START GUIDE
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EXTRA APPS
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MENU
Version Number_v25
1. Lathe Model
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2A. Design Your Gear Train
Number of compound gears in the gear train: 1
Manual entry of compound gears:
2B. Enter your own gears
List of change gears: 20,24,32,40,46,48,56,60
List of compound gears: 127/100,80,63
3. Calculation Type:
mm pitch, TPI, feed rate...
Calculation requested: mm pitch
4. Enter acceptable % error
Acceptable error in calculations: 2%
5A. Enter a single value
for mm pitch
mm pitch: 5
5B. Enter a range of values
for mm pitch
5C. Look up reference tables
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6. RUN 'A Ticket to Ride'
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HELP: SEARCH THE HELP FILE
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ELS: ELECTRONIC LEADSCREW.
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