Steel and things you need to know

In these page we are going to discuss about, What is Steel? Carbon Steel, Alloy Steel, Steel and its Constituents, High Strength Low Alloy Steel, Stainless Steel, Tool Steel, Properties of Some Steels, Production and Processing of Steel.

Introduction

So today we are going to talk about Materials especially Steel from Metals. Materials used in Engineering applications are broadly classified into six categories.

What is Steel ?

“Steel is iron-carbon alloys that may contain some amount of other alloy elements”. There are thousands of alloys that have different compositions and/or heat treatment processes.

❖    Plain Carbon Steels contain only residual concentrations of impurities other than carbon and a little manganese.

❖    For Alloy Steels, more alloying elements are intentionally added in specific concentrations.

 

Steels with carbon content from 0.025 to 0.8% are called as Hypoeutectoid Steels.

Steels with carbon content of 0.8% are called as Eutectoid Steels.

Steels with carbon content greater than 0.8% are called as Hypereutectoid Steels.

 

Carbon Steel

Carbon in steels may be present up to 2.0%. But some tell that it rarely exceeds 1.3 - 1.4%.

Some also tell that plain carbon steel is classified into 4 categories but not 3. Low, Medium, High, Ultra-High Carbon Steels.

Low Carbon Steels: It is also known as mild steel. It is a low-cost material. It’s reduced carbon content make it more malleable and ductile than other types of steel. While it is not as hard as higher carbon steels, carburizing can increase its surface hardness.

Medium Carbon Steels: Balanced Ductility and strength and has a good wear resistance. Used for large part, forging and automotive components, with long wearing properties.

High Carbon Steels: It is very strong and holds shape memory well, making it ideal for spring and wires.

Ultra - High Carbon Steels: Its high carbon content makes it an extremely stong material. Used for special purposes (non-industrial) like knives, axles, punches. This grade required special handling.

Most steels with more than 2.5% carbon are made using Powder Metallurgy.

“As the Carbon content of Steel Increases, it becomes Stronger and Harder. At the same time, it also becomes less Ductile”.

Carbon Steel	Carbon Percentage(%)	Microstructure	Properties	Examples
Low Carbon Steel	0.05 - 0.3%	Ferrite, Pearlite	Low hardness and low cost. High Ductility, Toughness, Machinability and Weldability	AISI 304, AISI 316L, ASTM A815.
Medium Carbon Steel	0.3 - 0.6%	Martensite	Low Hardenability, Medium Strength, Ductility and Toughness.	AISI 409, ASTM A29, SCM 435.
High Carbon Steel	0.6 - 1.0%	Pearlite	High Hardness, Strength and Low Ductility	AISI 440C, EN 10088-3.
Ultra - High Carbon Steel	1.0 - 2.0%	-	-	-

Alloy steel

It is the steel that is alloyed with a variety of elements in total amounts between 1.0 - 50% by weight to improve its mechanical properties. Alloy steel are classified into two categories Low Alloy Steel and High Alloy Steel.

Every steel is an alloy, but not all steels are called as “alloy steels”. The plain carbon steel are Fe + C about 0.1 to 1% depending on type.

In Alloy steel other elements are deliberately added to the carbon. 

Common alloys include Manganese, Nickel, Chromium, Molybdenum, Vanadium, Silicon and Boron these are more common. Aluminium, Cobalt, Copper, Cerium, Niobium, Titanium, Tungsten, Tin, Zinc, Lead and Zirconium are least common.

In alloy steels compared to carbon steels these properties are improved. Strength, Hardness, Toughness, Wear Resistance, Corrosion, Hardenability and Hot Hardness. To achieve some of these improved properties the metal may requires heat treating.

Low-alloy steels typically undergo heat treatment, normalizing and tempering during production. They are also weldable. However, weld heat treatment is necessary in order to avoid weld cracking.

Significant advantages of low-alloy steels over mild steel (Low Carbon Steel) are:

●     High yield strength

●     Able to withstand high temperatures

●     Good creep strength

●     Oxidation resistance

●     Hydrogen resistance

●     Low temperature ductility

A higher yield strength and creep strength is desirable because low-alloy steels are used to manufacture thin-walled pressure vessels.

Low-alloy steels such as 0.5 Mo and 12 Cr-Mo-V-W are used for their good creep properties in steam boilers, refinery crackers and reformers. The upper temperature limit for low-alloy steels is about 600°C (1112°F).

Steel and its Constituents:

An alloying element is one which is added to a metal to change properties.

Common alloying elements which are added to steel are:

C – Carbon; Ni – Nickel; Mo – Molybdenum; V – Vanadium; W – Tungsten; Mn – Manganese; Cu – Copper; Bo – Boron; Al – Aluminium; Co – Cobalt; Si – Silicon; Ti – Titanium; Cr - Chromium

Steel With Carbon:

●      Increases Hardness.

●      Increases Tensile Strength.

●      Lowers Machinability.

●      Lowers Melting Point.

Steel With Chromium:

●      Helps preventing corrosion and oxidation.

●      Adds strength at high temperature.

●      Joins with carbon to form chromium carbide, thus adds to depth hardenability with improved resistance to abrasion and wear.

Steel With Manganese:

●      Contributes remarkably to strength and hardness (But less when compared with Carbon).

●      Counteracts Brittleness from Sulphur.

●      Lowers both ductility and weldability, if it is present in high percentage with high carbon content in steel.

Steel With Molybdenum:

●      Promotes the Hardenability of Steel.

●      Makes steel fine grained.

●      Makes steel usually tough at various hardness levels.

●      Counteracts tendency towards temper brittleness.

●      Raises Tensile and Creep strength at high temperatures.

●      Enhances corrosion resistance in stainless steels.

●      Forms Abrasion resistance particles.

Steel With Silicon:

●      Improves oxidation resistance.

●      Strengthens low alloy steels.

●      Acts as a deoxidizer.

Steel With Titanium:

●      Prevents formation of Austenite in high chromium steels.

●      Prevents localised depletion of chromium in steels during long heating.

●      Reduces martensic hardness and hardenability in medium chromium steels.

Steel With Tungsten:

●      Increases hardness (and also red hardness).

●      Promotes fine grains.

●      Resists heat.

●      Promotes strength at elevated temperatures.

Steel With Vanadium:  

●      Promotes fine grain growth in steel.

●      Increase hardenability (when dissolved).

●      Imparts strength and toughness to heat treated steel.

●      Resits tempering and causes marked secondary hardening.

 

Production and Processing of Steel

Carbon steel can be produced from recycled steel, virgin steel or a combination of both.

Virgin steel is made by combining iron ore, coke (produced by heating coal in the absence of air) and lime in a blast furnace at around 1650°C. The molten iron extracted from the iron ore is enriched with carbon from the burning coke. The remaining impurities combine with lime to form slag, which floats on the top of the molten metal where it can be extracted. 

The resulting molten steel contains roughly 4% of carbon. This carbon content is then reduced to the desired amount in a process called decarburization. This is achieved by passing oxygen through the melt, which oxidizes the carbon in the steel, producing carbon monoxide and carbon dioxide.

High Strength Low Alloy Steel

Another group of Low Carbon alloys are called High Strength Low Alloy (HSLA) steels.

They contain other alloying elements such as Copper, Vanadium, Nickel and Molybdenum in combined concentrations as high as 10%.

They possess higher strengths than the plain low carbon steels. Most may be strengthened by heat treatment, giving Tensile Strength in excess of 480 Mpa (70,000 psi)

In normal atmospheres, the HSLA steels are more resistant to corrosion than the plain carbon steels, which they have replaced in many applications where Structural Strength is Critical.

Example: Bridges, Towers, Support Columns in high rise buildings and Pressure Vessels.

HSLA are Ductile, Formable and Machinable.

Stainless Steel

When 11% or more chromium is added to iron, a fine film of Chromium oxides forms on the surface exposed to air. This film acts as a barrier to retard further oxidation, rust or corrosion. This steel cannot be stained easily and this steel is called Stainless Steel.

Stainless Steel can be grouped into three metallurgic classes.

1. Austenitic Stainless Steel.
2. Ferritic Stainless Steel.
3. Martensitic Stainless Steel.

Austenitic Stainless Steel.

●      They possess Austenitic Structure at room temperature.

●      Highest corrosion of all stainless steels.

●      They retain ductility at temperatures approaching absolute zero.

●      They are non-magnetic so that they can be easily identified with a magnet.

●      Composition:

○      C: 0.03 to 0.25%;

○      Mn: 2 to 10%;

○      Si: 1 to 2%;

○      Cr: 16 to 26%;

○      Ni: 3.5 to 22%;

○      Phosphorus and Sulphur normal.

●      Applications: Engine parts of Aircrafts, Heat exchangers, Kettles, Tanks, Cooking Utensils, Milk Cans, Trailers and Railway cars.

 

Ferritic Stainless Steel.

●      They possess Microstructure which is primarily Ferritic.

●      They have Low Carbon to Chromium ratio. This eliminates the effects of thermal transformation and prevents hardening by heat treatment.

●      They are Magnetic and have good ductility, more corrosive resistant than martensitic stainless steel.

●      They develop maximum softness, ductility and corrosion resistance by annealing.

●      These steels do not work harden to any appreciable degree.

●      Composition:

○      C: 0.08 to 0.2%;

○      Mn: 1 to 1.5%;

○      Si: 1%;

○      Cr: 11 to 27%.

 

Martensitic Stainless Steel.

●      They are identified by their martensitic microstructure in the hardened condition.

●      They have high carbon to chromium ratio, hence the only types hardenable by heat treatment.

●      These steels are magnetic in all conditions and have the best thermal conductivity of all the stainless steel.

●      Hardness, Ductility and ability to hold an edge are characteristics.

●      They can be cold worked without difficulty. Can be machined satisfactorily, have good toughness, show good corrosive resistance to weather and are easily hot worked.

●      Composition:

○      C: 0.15 to 1.2%;

○      Mn: 1%;

○      Si: 1%;

○      Cr: 11.5 to 18%.

Tool Steel

Tool and Die steel may be defined as special steels which have been developed to form, cut or otherwise change the shape of materials into a finished or semi-finished product. Their properties are:

• Slight change of form during hardening.
• Little risk of cracking during hardening.
• Good toughness, wear resistance.
• Very good machinability.
• A definite cooling rate during hardening, hardening temperature.
• A good degree of through hardening.
• Resistance to decarburization.
• Resistance to softening of heating (red hardness).

 

Tool Steel Classifications

Water Hardening Tool Steels (W – Series)

Essentially these are carbon steels with 0.60 to 1.10% carbon.

Lowest cost tool steels.

Soft core (for toughness) with hard shallow layer (for wear resistance).

Use of W-series steels is declining

Oil Hardening Tool Steels (O – Series)

They contain 0.90 to 1.45% carbon with Mn, SI, W, Mo, Cr.

They contain graphite in the hardened structure along with martensite. Graphite acts as a lubricator and also makes machining easier.

Tungsten forms tungsten carbide which improves the abrasion resistance and edge retention in cutting devices.

 

Medium Alloy Air Hardening Steels (A – Series)

They contain 5 to 10% alloying elements Mn, Si, W, Mo, Cr, V, Ni to improve the hardenability, wear resistance, toughness.

 

High Carbon High Chromium Steels (D – Series)

All D – Series contain 12% Cr and over 1.5% C.

Air or oil quench.

Low distortion, high abrasion resistance.

 

Heat Treatment of Tool Steels

·       Carbon and low alloy steels may be quenched in water or brine solution and high alloy steels in oil, air or molten salts.

·       Overheating of Tool steel should be avoided.

·       Protective furnace atmosphere or any other method should be employed to avoid scaling or decarburization of tool steel during heating.

·       Tool steels should be heated slowly to the desired heat treatment temperatures.

·       Tool steels should be kept at proper temperature for sufficient time so that the whole of the tool section gets uniformly heated.

·       Tool steels should be tempered immediately after quenching and before they cool to room temperature. This reduces possible cracking.

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