For applications requiring higher tensile strengths and toughness than the carbon steels there is a range of low alloy steels. These are categorised as high tensile or constructional steels and case hardening steels. The high tensile strength steels have sufficient alloying additions enabling through hardening (by quench and temper treatment) according to their alloying additions.

Case Hardening (carburising) Steels

Case hardening steels are a group of low carbon steels in which a high hardness surface zone (hence the term ‘case hardened’) is developed during heat treatment by absorption and diffusion of carbon. The high hardness zone is supported by the unaffected underlying core zone, which is lower hardness and higher toughness.

Plain carbon steels that can be used for case hardening are restricted. Where plain carbon steels are used, the rapid quenching necessary to develop satisfactory hardness within the case can cause distortion and the strength that can be developed in the core is very limited. Alloy case hardening steels allow the flexibility of slower quenching methods to minimise distortion and high core strengths can be developed.

Nitriding steels can have higher surface hardness developed by absorption of nitrogen, when exposed to a nitriding atmosphere at temperatures in the range of 510-530ºC, after hardening and tempering.

High tensile steels suitable for nitriding are: 4130, 4140, X4150, 4340, En25, 8660, En26.

Chemical Composition (% by weight - nominal values) 

Grade	C	Si	Mn	Cr	Mo	S	Ni
High Tensile
4140	0.40	-	0.80	0.90	0.20	-	-
X4150	0.50	-	1.30	0.65	0.18	0.08	-
4340	0.40	-	0.80	0.80	0.25	-	1.80
En25	0.30	-	0.60	0.65	0.55	-	2.50
En26	0.40	-	0.60	0.65	0.55	-	2.50
Case Hardening
En36A	0.12	-	0.50	0.90		-	3.25
En39B	0.15	-	0.50	1.20	0.20	-	4.20
8620	0.20	-	0.80	0.50	0.20	-	0.50
6687	0.16	-	0.50	1.65	0.30	-	1.55

 

Minimum Tensile Strengths 

Grade	Specification
4140	Up to 50mm AS 1444, Condition T
	50 to 100mm AS 1444, Condition T
	Over 100mm ASTM A434, Class BD
X4150	Up to 100mm AS2506, Condition U
	Above 100mm ASTM A434 Class BD, with composition to AS2506, X4150
4340/6582	Up to 100mm AS 1444, Condition U
	Above 100mm ASTM A434, 4340 Class BD
En25	AS 1444, Condition U or BS 970: Part 3, 826 M31, Condition U
En26	BS 970: Part 3, 826M40, Condition W.

 

Note 1: Specified hardenability grades are available on request.

Note 2: The actual specification of the product ordered will depend upon the product’s origin – for deviations from the above specification refer theto the Technical Department.

Note 3: Alloy bar can be sourced in alternative conditions, i.e. annealed or other hardened and tempered conditions.

 

AS 1444/BS 970 tensile strength designation		ASTM A434 Class BD tensile strength
Tensile strength designator	Tensile strength MPa	Diameter (mm)	Tensile strength Min. MPa
R	700-850	38.1 and under	1070
S	770-930	over 38.1 to 63.5 inclusive	1030
T	850-1000	over 63.5 to 114 inclusive	960
U	930-1080	over 114 to 178 inclusive	930
V	1000-1150	over 178 to 241.3 inclusive	900
X	1080-1230
X	1150-1300
Y	1230-1380
Z	1550 min.

Yield (proof) stress and elongation limits also apply – refer to AS 1444, BS 970 or ASTM A434.

 

Typcial Applications 

Grade	Description
4140	General-purpose high tensile, used for axles, shafts, high tensile studs and bolts, gears and drill rods. The steel is also suitable for flame and induction hardening.
x4150	A free machining high tensile steel suitable for highly stressed components needing considerable toughness. Supplied in the heat-treated condition. Can be welded by standard methods, preheating and post-heating usually required. Used in machine tools, transportation equipment, excavating and road machinery, draw bench chain, oil drilling, nuts and bolts.
4340/6582	Small highly stressed components such as gears, an alternative to En25 and En26 in smaller sizes.
6580	Atlas 6580 alloy bar is the ultimate through hardening steel that offers exceptional yield and tensile strengths whilst maintaining good toughness and ductiility, an alternative to En25 and En26 grades.
En25	En25 is used extensively in most industry sectors for applications requiring higher tensile and yield strength than 4140 can provide, similiar to En26 but with lower carbon content.
En26	Characterised by high strength and toughness in very large sections, similar to En25 but with a higher carbon content. En26 is used extensively in most industry sectors for applications requiring higher tensile strength and yield strength than 4140, 4340 or En25 can provide.
Case Hardening
8620	General-purpose case hardening steel suitable for comparatively lightly stressed components. Used for automatic components such as transmission gears, crown wheels, ring gears, hypoid gears and racers, king pons and pinions.
Atlas 6587	The high chromium content produces high case hardness with excellent core strength. Used for gears, bearings, sleeves, pins, bushes, shafts, plastic moulds and any highly stressed or high wear component.
En36A	Used extensively for highly stressed gears in trucks, agriculture and mining machinery, pins and brushes, heavily loaded shafts and other applications requiring a hard surface with a tough shock-resisting core. Core tensile 55tsi minimum after hardening and tempering.
En39B	An exceptionally high hardenability steel for large or highly stressed gears where core strength and toughness is required. May be used as an alloy engineering steel.Core tensile strength 85tsi minimum after hardening and strengthening.