Alloy steel vs Carbon steel?

Question

What are the advantages and disadvantages of alloy steel over carbon steel?

Answer 1

Pure iron is relatively soft and adding small amounts of carbon hardens the 'carbon steel' by affecting solid crystals.  Larger amounts of carbon (cast iron) will make the metal even harder but relatively brittle.

Other alloying agents (often in addition to the correct amount of carbon) will provide desirable properties.  For example, leaded steel is more easily machined.  Nickle may be added to enhance magnetic properties.  Chrome may be added to reduce corrosion (stainless steels).

In addition to the amount of carbon used in steels, heat treatment can produce desirable properties.  Heating the steel helps dissolve the carbon and quenching (rapid cooling) keeps the carbon evenly distributed.  Slow cooling will allow crystals to grow pushing the excess carbon to the final grain boundaries and the characteristics of the steel will depend upon grain sizes and strength of the grain boundaries.

Answer 2

Alloy steel is required for special application, mainly harsh operating environment, H2S high contamination, etc

Answer 3

Plain carbon steels have some draw backs which limits their use to under relatively non-severe conditions. There major draw backs are:

1. Low strength in high temperatures

2. Low resistance to corrosion and oxidation at high temperatures

3. Low hardenability

4. Significant drop of hardness on tempering after hardening

To overcome such drawbacks and to broaden their usefulness, alloy steels have been developed by deliberately adding alloying elements (chemical elements) to steels. Therefore, most of the steels, actually all the steels, used in severe conditions are alloy steels today. E.g: Highly fatigue resistant alloys have been developed for high speed applications like motor vehicle shafts called HSS ( High Speed Steels).

However, generally alloying induces some brittleness into steels which maybe identified as their weakness. But, many remedies have been introduced to avoid this situation and better alloys have been developed. Therefore, there's hardly any disadvantage (may be the cost) in Alloys over carbon steels.