Difference Between Low-Alloy Steel & High-Alloy Steel

Alloy Steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low-alloy steels and high-alloy steels.

High-Alloy Steel

High-alloy steels are defined by a high percentage of alloying elements. Stainless steel is high-alloy steel which contains at least 12 percent chromium. Stainless steel is devided into three basic types: martensitic, ferritic, and austenitic. Martensitic steels contain the least amount of chromium, and known for high hardenability, and are mainly used for cutlery. Ferritic steels contains 12 to 27 percent nickel chromium alloy and are suitable to use in automobiles and industrial equipment. Austenitic steels contain high levels of nickel, carbon, manganese, or nitrogen and usually have the highest corrosion resistance. Austenitic steels are mainly used to store corrosive liquids and mining, chemical, or pharmacy equipment.

Low-Alloy Steel

Among alloy steels, when Ni, Cr, Mo, and other alloy elements content consist of less than 10.5% are known as low alloy steels. Low-alloy steels have a much lower percentage of alloying elements, usually 1 to 5 percent. These steels have very different strengths and uses depending on the chosen alloy. High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbonsteels in the normal sense.

Alloy steel can further classify as

Low alloy steels: in which sum of total Alloying elements is < 5%
High alloy steels: in which sum of total Alloying elements is > 5%

Uses of Alloy Steel

Use of Alloy Steel is mainly in automobile industry and machinery parts. Alloy steel can be used in process area where carbon steel has limitation, below are few application of alloy steel:

High-temperature services such as Heater tubes, Alloy Steel Boiler Tubes
Low-temperature services such as Cryogenic application
Very High presser service such as Steam Header
Used in construction of aircrafts and heavy vehicles for crank shafts, camshafts and propellor shafts, etc.
General Engg and mould basis.

Here you can see the common alloy steel grade that you will come across.

For Pipes: ASTM A335 Gr P1, P5, P11, P9
For Wrought Fittings: ASTM A234 Gr.WP5, WP9, WP11
For Forged Fittings: ASTM A182 F5, F9, F11 etc.

Alloy steel grades

4340
Specs: AMS 6359, AMS 6414, AMS 6415
Form: Flat Bar, Plate, Round Bar
300M
Specs: AMS 6257, MIL-S-8844, AMS 6417, AMS 6419, AMS 2300, BMS 7-26 Type 1, GE S400, GE S1000
Form: Flat Bar, Round Bar
4330 M
Specs: AMS 6411, AMS 6427, BMS 7-122, AMS 2300, GE S400, GE S1000
Form: Flat Bar, Round Bar
9310
Specs: AMS 6265, AMS 6260
Form: Round Bar
HP9-4-30
Specs: AMS 6526 (except N&T), EMS 96247
Form: Round Bar, Forged Bar
NITRIDING 135
Specs: AMS 6470, AMS 6471, AMS 6472
Form: Round Bar
H11
Specs: AMS 6485, AMS 6487, AMS 6488
Form: Round Bar

Alloy steel properties

	MATERIAL			CHEMISTRY								Application
AISI/ASTM	EN	DIN	JIS	C	SI	Mn	P	S	Cr	W	Mo
4340	EN24	40NiCrMo84		0.40	0.25	0.65	0.03	0.04	1.2	1.5	0.37	Used in construction of aircrafts and heavy vehicles for crankshafts, gearshafts, camshafts and propellor shafts etc.
52100	EN31	100Cr6		0.45	0.25	0.8	0.03	0.03				General engineering and mould basis.
	EN353	20NiCrMo5		.20 MAX	.35 MAX	.50 - 1.00	.040	.040	.75 - 1.25		.08 - .15	Machining components.
8620	S620	21NiCrMo2		0.20	0.80	1.70	0.020-0.025	0.010-0.015	1.50		0.70	Machining components.
4140	4140	42CrMo4		.38 - .43	.20 - .35	.75 - 1.00	.035	.035	.80 - 1.10			Machining components.
	16Mncr5	16MnCr5		.14 - .19	.15 - .40	1.00 -1.30	.035 MAX	.035 MAX	.80- 1.10			Machining components.
5120	20Mncr5	20MnCr5		.17 - .22	.10 - .35	1.10 - 1.40	.035	.035	1.00 - 1.30			Machining components.

Alloying Elements

Commonly used alloying elements are

Chromium – Increases Resistance to corrosion and oxidation. Increases harden ability and wear resistance. Increases high temperature strength.
Nickel – Increases harden ability. Improves toughness. Increases impact strength at low temperature.
Molybdenum – Increases harden ability, high temperature hardness and wear resistance. Enhances the effects of other alloying elements. Eliminate temper brittleness in steels. Increases high temperature strength.
Manganese – Increases harden ability. Combines with sulphur to reduce its adverse effects.
Vanadium – Increases harden ability, high temperature hardness and wear resistance. Improves fatigue resistance.
Titanium – Strongest carbide former. Added to stainless steels to prevent precipitation of chromium carbide.
Silicon – Removes oxygen in steel making. Improves toughness. Increases hardness ability
Boron – Increases harden ability. Produces fine grain size.
Aluminium – Forms nitride in nitriding steels. Produces fine grain size in casting. Removes oxygen in steel melting.
Cobalt – Increases heat and wear-resistance.
Tungsten – Increases hardness at elevated temperatures. Refines grain size.

Role of alloying elements

Depending on the quantities of alloying elements following properties of material get affected such as

Corrosion resistance
Hardenability
Machinability
High or low-temperature Stability
Ductility
Toughness
Better Wear resistance
Improved Weldability

Type by Alloy

Alloy steel is often categorized based on the type of alloy and its concentration. These are a few of the most common additions to alloy steel:

Aluminum removes oxygen, sulfur, and phosphorus from steel.
Bismuth improves machinability.
Chromium increases wear resistance, hardness, and toughness.
Cobalt increases stability and encourages the formation of free graphite.
Copper improves hardening and corrosion resistance.
Manganese increases hardenability, ductility, wear resistance, and high-temperature strength.
Molybdenum lowers carbon concentration and adds room-temperature strength.
Nickel improves strength, corrosion resistance, and oxidation resistance.
Silicon increases strength and magnetism.
Titanium improves hardness and strength.
Tungsten improves hardness and strength.
Vanadium increases toughness, strength, corrosion resistance, and shock resistance.

Is carbon steel or alloy steel stronger?

Carbon steel is not stainless steel as it is classified under alloy steels. As the name implies, carbon content is increased in the steel making it harder and stronger through application of heat treatments. However, addition of carbon makes the steel less ductile.

What is meant by non alloy steel?

These steels are characterized mainly by their utilizisation mechanical properties. ... They principally consist of non alloy or alloy steels containing boron, manganese, silicium, nickel, chromium, molybdenum or a combination of these elements.

What are the different types of alloy steel?

However, the term "alloy steel" is the standard term referring to steels with other alloying elements added deliberately in addition to the carbon. Common alloyants include manganese (the most common one), nickel, chromium, molybdenum, vanadium, silicon, and boron.

How strong is alloy steel?

Stainless Steel – An alloy of steel, chromium and manganese produces a corrosion-resistant metal with yield strength of up to 1,560 Mpa and a tensile strength of up to 1,600 Mpa. Like all types of steel, this alloy is highly impact resistant and scores mid-range on the Mohs scale.

What is the world's strongest metal alloy?

New Magnesium based alloy as World's strongest and lightest metal to change the world. Researchers from North Carolina State University have developed a material using magnesium which is light like aluminum, but as strong as titanium alloys. This material has the highest strength-to-weight ratio known to mankind.

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