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Machining aspects of alloy steel

There is no scientific definition, but in practice, alloy steels are carbon steels with additional alloying elements (on top of the carbon and Manganese) of up to 5%. These elements are added to improve the strength, toughness, corrosion resistance, wear resistance, hardenability, and the steel’s hot hardness.

Introduction

The simplest and cheapest form of steel used in mechanical components is carbon steel. For various general-purpose components, it is good enough; however, in many cases, for a mechanical part to function, it needs specific properties such as wear resistance, strength, hardness, hot hardness, etc.
Thankfully, adding certain alloying elements to carbon steel yields alloys with better mechanical and metallurgical characteristics.
Don’t forget the Carbon!

Don’t forget the Carbon!

As important as all these alloying elements are, Carbon content is still the primary factor determining steel mechanical properties and machinability. Therefore, before you dive into this article, we recommend you be familiar with our article about carbon steel.

Classification System

The most common classification system for alloy steel is the ASTM/SAE system based on a 4 digits format.
For example: 4340, 5132, and 8630.

  • Digit 1 – Series number and Main classification. For example, 3xxx is a series of Nickel-Chromium alloys and 8xxx of Nickel-Chromium-Molybdenum alloys.
  • Digit 2 – Amount of total alloying elements. It is not quantitative. But a higher number means a larger amount of the sum of all the alloy elements. For example, 31xx has 1.9% nickel and chromium (combined quantity), and 34xx has 3.7%.
  • Digits 3 and 4 – Carbon amount. For example 4130 has 0.3% carbon and 4140 has 0.4% carbon.


Alloy steel serieses

Series (SAE/ASTM) Description Molybdenum Chromium Nickel Tungsten Silicon Vanadium
1xxx Plain Carbon Steel - More details in a Separate Article
2xxx Nickel Steel 3.5-5.0%
3xxx Nickel-Chromium Steel 0.6-1.6% 1.2-3.0%
4xxx For 4xxx series see details in the Below Table
5xxx Chromium Steel 0.5-1.5%
6xxx Chromium-Vanadium Steel 0.6-0.9% 0.1-0.15%
7xxx Tungsten-Chromium Steel 0.7-0.8% 1.7-1.8%
8xxx Nickel-Chromium-Molybdenum Steel 0.1-0.4% 0.4-0.5% 0.3-0.6
92xx Silicon-Chromium Steel 0.6-0.7% 1.4-2.0%
93xx-98XX Nickel-Chromium-Molybdenum Steel 0.1-0.25 0.2-1.2% 0.5-3.3%

4xxx Series

The 4xxx is the most common alloy steel in machine shops and it has seven sub-serieses
Series (SAE/ASTM) Description Molybdenum Chromium Nickel
40xx Molybdenum steel 0.2-0.5%
41xx Chromium-molybdenum Steel 0.1-0.3% 0.4-0.6%
43xx Nickel-chromium-molybdenum steel 0.2-0.3% 0.5-0.8% 1.7-1.9%
44xx Molybdenum steel 0.4-0.5%
46xx Nickel-molybdenum steel 0.2-0.3% 0.9-1.8%
47xx Nickel-chromium-molybdenum steel 0.2-0.3% 0.4-0.5% 1.0-1.1%
48xx Nickel-molybdenum steel 0.2-0.3% 3.4-3.6%

Alloying elements

  • Important note: The information below refers to the addition of alloying elements in small quantities relevant to alloy steel only! The same alloying elements can have a completely different effect when added in higher doses. For example, when adding chromium and nickel in higher quantities, the steel becomes austenitic stainless steel. Check out our separate article to learn about the broader effects of Alloying Elements 

Alloying elemnts effects on machining and material properties

Alloying Element Effect on Properties Effect on Machining
Molybdenium [Mo]

Improves:

  • Hardenability
  • Strength
  • Creep Strength
  • High Temperature

Damages:

  • Weldability
  • Not significant.
Chromium [Cr]
Improves:
  • Strength.
  • Hardenability.
  • Scratch resistance (Above 1%).
  • High-temperature strength (Above 1%).
  • More difficult to achieve chip control
  • Decreases machinability
Nickel [Ni]
Improves:
  • Strength.
  • Hardenability.
  • Toughness.
  • More difficult to achieve chip control
  • Decreases machinability
Tungsten [W]
Improves:
  • Strength.
  • Hardness.
  • Toughness.
  • Wear resistance.
  • Hot Hardness.
  • Decreases machinability
Silicon [Si]
Improves:
  • Deoxidation.
  • Strength.
  • Sag resistance (Above 1%).
  • Faster wear of the cutting edge
Vanadium [V]

Improves:

  • Deoxidation.
  • Strength.
  • Wear resistance.
  • Shock resistance.

Damages:

  • Ductility

  • Slight Decrease in machinability
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