Selected Metals - 2:Alloy Steel
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
Alloy steels are those carbon steels that have certain other elements purposefully mixed in during processing to achieve various results in the performance of the steel. As mentioned in the previous page there are sometimes unintentional alloys in steel.. These impurities can be present in the iron ore or can be added for refining purposes but they are not included to manipulate the physical characteristics of the steel. For example the problem with the iron ore in the British Isles is that it contains too much phosphorus which makes the steel brittle. The solution - circa 1800 - was to add chalk to the molten metal* which caused the phosphorous to mix with the slag and the slag could be easily removed in the steel making process. This steel, should it contain some residual chalk residue, is not an alloy steel because no alloys were intentionally added to the steel to change the properties. Only to make the refining easier.

The first alloy steel was invented by Robert Hadfield in Sheffield England. By mixing large quantities of manganese with the iron and carbon he made an alloy steel that was extremely hard wearing and most suitable for train rails. For many decades this steel was known as "manganese steel". Note that manganese was already used in the steel making process and was limited as an impurity to no more than 0.7% content. Hadfields manganese steel had about 1.6 % manganese.

Many other alloying elements have followed which produce steels with greater strength, fatigue resistance, corrosion resistance, and other physical attributes. Alloy elements are commonly chromium, molybdenum, manganese, and nickel.  Following are three molybdenum alloy steels that are familiar to most machinists (note how carbon content and alloy effect their properties).

N41300 (AISI/SAE 4130)

  • Carbon 0.25 to 0.35%
  • Alloy molybdenum and chromium
  • Machinability medium plus when normalized
  • Hadenability very good. Should not be case hardened
  • Weldability good by all methods
  • Application aircraft engine mountings and welded tubing.
  • Comments a good, strong, machinable, steel that is available from all suppliers and in most shapes.

N41400 (AISI/SAE 4140)

  • Carbon 0.38 to 0.43%
  • Alloy molybdenum, manganese, chromium,
  • Machinability very good
  • Hadenability good oil hardened steel
  • Weldability good
  • Applications nearly everything
  • Comments a common low alloy steel noted for toughness, good torsional strength and good fatigue strength. This alloy steel is the most common and is available in nearly all sizes and shapes.

N43300 (AISI/SAE 4330)

  • Carbon 0.28 to 0.32%
  • Alloy molybdenum, nickel, chromium,
  • Machinability good only if annealed
  • Hadenability good oil hardened steel
  • Weldability good by all methods but requires pre/post heating for large sections
  • Applications gears, aircraft landing gear, axles or shafts for power transmission
  • Comments a common low alloy steel noted for toughness, good torsional strength and good fatigue strength. This alloy steel is quite common and is available in nearly all sizes and shapes

"Free machining" is a common term used in the machine shop to categorize the steels that are the easiest to machine.

Free Machining steels are relatively low carbon steels that have been resulfurized to make steel that produces better chips (chip fracturing) and requires less horsepower to machine. R.S.

A peculiar aside --- AISI/SAE steel designations determine that any steel that begins with the designation numeral "1" is classified as a carbon steel, not an alloy steel. Alloy steels begin with some other digit, such as "2" for nickel steel and "4" for molybdenum steel. Therefore any number other than "1" designates a steel with some primary alloying element. Yet manganese steel does not have its own numeral at the beginning of the designator. Instead, manganese steel, is a subset of carbon steel with a designator of 13xx where the "13" designates this carbon steel as a manganese steel...go figure. R. S.

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