Heat treatment

4140 is hardened from 840 to 875 °C, quenched in oil and tempered between 550 and 680 °C: the span inside all three data sheets read. Carburising 4 hours at 925 °C gives about 1.26 mm of total case. Pick a grade for each source’s hardening and tempering range side by side, a tempering temperature for its temper colour, or a carburising time for the case depth. The supplier’s data sheet for your bar governs.

The cycle is drawn as temperature against time, every source’s range as a band and the part they share solid.

Grade

Tempering at

The supplier's data sheet for your bar governs: these are published ranges, and mills differ for the same grade.

Harden: 840 – 875°C
Temper: 550 – 680°C

Quench: Oil

How it’s calculated

$d = \frac{803 t}{1 0 ^{3722/ T}}$ F. E. Harris’s total case depth d in mm after t hours at T kelvin (°C + 273.15), for plain carbon steel: 0.63 mm per √h at 925 °C. Its inch form, 31.6 √t / 10^(6700 / T °R), gives 0.031 in for 1.5 h at 1700 °F.

Grade	Source	Harden, °C	Temper, °C
1045	Interlloy	820 – 860	400 – 650
	West Yorkshire Steel	820 – 860	550 – 660
	Ovako	820 – 870	—
4140	Interlloy	840 – 875	550 – 700
	West Yorkshire Steel	840 – 875	550 – 700
	Ovako	840 – 880	540 – 680
4340	Interlloy	830 – 860	450 – 660
	West Yorkshire Steel	830 – 865	—
	Ovako	820 – 850	540 – 680
8620	Interlloy	780 – 820	150 – 200
	West Yorkshire Steel	—	—
	Ovako	780 – 830	160 – 250
O1	West Yorkshire Steel	780 – 820	100 – 350
	Uddeholm	790 – 850	180 and up
	Böhler	780 – 820	—
A2	West Yorkshire Steel	950 – 980	—
	Uddeholm	925 – 970	180 and up
	Böhler	950 – 980	—
D2	West Yorkshire Steel	1000 – 1040	—
	Uddeholm	990 – 1080	180 and up
	Böhler	1030 – 1070	—
H13	West Yorkshire Steel	1000 – 1030	530 – 650
	Uddeholm	1020 – 1050	250 and up
	Böhler	1020 – 1080	—
420	Carpenter	1010 – 1066	149 – 204
	AK Steel	982 – 1066	—
	West Yorkshire Steel	980 – 1050	250
	Böhler	1025 – 1050	250 – 350
440C	Carpenter	1010 – 1066	149 – 427
	West Yorkshire Steel	1010 – 1070	150 – 400
17-4PH	ATI	1040 – 1065	aged
	West Yorkshire Steel	1038	aged

Temper colour	West Yorkshire Steel, °C	Navy, °C
Light yellow / pale yellow	210	220
Straw	220	230
Dark yellow / golden yellow	240	243
Yellow brown / brown	250	254
Red brown / brown dappled with purple	260	266
Purple	270	277
Dark blue	290	288
Wedgewood blue / bright blue	300	297
Light blue / pale blue	310	321

Why do the data sheets disagree?

A grade name covers a chemistry range, and each mill sets its own window for its own melt: Böhler hardens D2 from 1030 °C, West Yorkshire Steel says not to exceed 1040 °C. Tool steel mills also temper to a chart for the hardness wanted rather than to one range. So the page gives every source’s figure and the span they share, and the sheet that came with the bar is the one to follow.