Carbon steel

Carbon steel

Carbon steel is asteel with carbon content from about 0.05 up to 2.1 percent by weight. Thedefinition of carbon steel from the American Iron and Steel Institute (AISI)states:

no minimumcontent is specified or required for chromium, cobalt, molybdenum, nickel,niobium, titanium, tungsten, vanadium, zirconium, or any other element to beadded to obtain a desired alloying effect;

the specifiedminimum for copper does not exceed 0.40%;

or the maximumcontent specified for any of the following elements does not exceed thepercentages noted: manganese 1.65 per cent; silicon 0.60 per cent; copper 0.60per cent.[1]

The term carbonsteel may also be used in reference to steel which is not stainless steel; inthis use carbon steel may include alloy steels. High carbon steel has manydifferent uses such as milling machines, cutting tools (such as chisels) andhigh strength wires. These applications require a much finer microstructure,which improves the toughness.

Carbon steel is apopular metal choice for knife making due to its high amount of carbon, givingthe blade more edge retention. To make the most out of this type of steel it isvery important to heat treat it properly. If not the knife may end up beingbrittle, or too soft to hold an edge.

As the carbonpercentage content rises, steel has the ability to become harder and strongerthrough heat treating; however, it becomes less ductile. Regardless of the heattreatment, a higher carbon content reduces weldability. In carbon steels, thehigher carbon content lowers the melting point.

Mild orlow-carbon steel

Mild steel (ironcontaining a small percentage of carbon, strong and tough but not readilytempered), also known as plain-carbon steel and low-carbon steel, is now themost common form of steel because its price is relatively low while it providesmaterial properties that are acceptable for many applications. Mild steelcontains approximately 0.05–0.30% carbonmaking it malleable and ductile. Mildsteel has a relatively low tensile strength, but it is cheap and easy to form;surface hardness can be increased through carburizing.

In applicationswhere large cross-sections are used to minimize deflection, failure by yield isnot a risk so low-carbon steels are the best choice, for example as structuralsteel. The density of mild steel is approximately 7.85 g/cm3 (7850 kg/m3 or0.284 lb/in3) and the Young's modulus is 200 GPa (29,000 ksi).

Low-carbon steelsdisplay yield-point runout where the material has two yield points. The firstyield point (or upper yield point) is higher than the second and the yielddrops dramatically after the upper yield point. If a low-carbon steel is onlystressed to some point between the upper and lower yield point then the surfacedevelops Lüder bands. Low-carbon steels contain less carbon than other steelsand are easier to cold-form, making them easier to handle. Typical applicationsof low carbon steel are car parts, pipes, construction, and food cans.

High-tensilesteel

High-tensilesteels are low-carbon, or steels at the lower end of the medium-carbonrange,[citation needed] which have additional alloying ingredients in order toincrease their strength, wear properties or specifically tensile strength.These alloying ingredients include chromium, molybdenum, silicon, manganese,nickel, and vanadium. Impurities such as phosphorus and sulfur have theirmaximum allowable content restricted.

Higher-carbonsteels

Carbon steelswhich can successfully undergo heat-treatment have a carbon content in therange of 0.30–1.70% by weight. Trace impurities of various other elements canhave a significant effect on the quality of the resulting steel. Trace amountsof sulfur in particular make the steel red-short, that is, brittle and crumblyat working temperatures. Low-alloy carbon steel, such as A36 grade, containsabout 0.05% sulfur and melts around 1,426–1,538 °C (2,599–2,800 °F). Manganeseis often added to improve the hardenability of low-carbon steels. Theseadditions turn the material into a low-alloy steel by some definitions, butAISI's definition of carbon steel allows up to 1.65% manganese by weight.

High-carbon steel

Approximately 0.6to 1.0% carbon content. Very strong, used for springs, edged tools, andhigh-strength wires.

Ultra-high-carbonsteel

Approximately1.25–2.0% carbon content.[ Steels that can be tempered to great hardness. Usedfor special purposes like (non-industrial-purpose) knives, axles, and punches.Most steels with more than 2.5% carbon content are made using powdermetallurgy.

Heat treatment

Iron-carbon phasediagram, showing the temperature and carbon ranges for certain types of heattreatments.

Main article:Heat treatment

The purpose ofheat treating carbon steel is to change the mechanical properties of steel,usually ductility, hardness, yield strength, or impact resistance. Note thatthe electrical and thermal conductivity are only slightly altered. As with moststrengthening techniques for steel, Young's modulus (elasticity) is unaffected.All treatments of steel trade ductility for increased strength and vice versa.Iron has a higher solubility for carbon in the austenite phase; therefore allheat treatments, except spheroidizing and process annealing, start by heatingthe steel to a temperature at which the austenitic phase can exist. The steelis then quenched (heat drawn out) at a moderate to low rate allowing carbon todiffuse out of the austenite forming iron-carbide (cementite) and leaving ferrite,or at a high rate, trapping the carbon within the iron thus forming martensite.The rate at which the steel is cooled through the eutectoid temperature (about727 °C) affects the rate at which carbon diffuses out of austenite and formscementite. Generally speaking, cooling swiftly will leave iron carbide finelydispersed and produce a fine grained pearlite and cooling slowly will give acoarser pearlite. Cooling a hypoeutectoid steel (less than 0.77 wt% C) resultsin a lamellar-pearlitic structure of iron carbide layers with α-ferrite (nearlypure iron) between. If it is hypereutectoid steel (more than 0.77 wt% C) thenthe structure is full pearlite with small grains (larger than the pearlitelamella) of cementite formed on the grain boundaries. A eutectoid steel (0.77%carbon) will have a pearlite structure throughout the grains with no cementiteat the boundaries. The relative amounts of constituents are found using thelever rule. The following is a list of the types of heat treatments possible:

Spheroidizing

Spheroidite formswhen carbon steel is heated to approximately 700 °C for over 30 hours.Spheroidite can form at lower temperatures but the time needed drasticallyincreases, as this is a diffusion-controlled process. The result is a structureof rods or spheres of cementite within primary structure (ferrite or pearlite,depending on which side of the eutectoid you are on). The purpose is to softenhigher carbon steels and allow more formability. This is the softest and mostductile form of steel.

Full annealing

Carbon steel isheated to approximately 40 °C above Ac3 or Acm for 1 hour; this ensures all theferrite transforms into austenite (although cementite might still exist if thecarbon content is greater than the eutectoid). The steel must then be cooledslowly, in the realm of 20 °C (36 °F) per hour. Usually it is just furnacecooled, where the furnace is turned off with the steel still inside. Thisresults in a coarse pearlitic structure, which means the "bands" ofpearlite are thick.Fully annealed steel is soft and ductile, with nointernal stresses, which is often necessary for cost-effective forming. Onlyspheroidized steel is softer and more ductile.

Process annealing

A process used torelieve stress in a cold-worked carbon steel with less than 0.3% C. The steelis usually heated to 550–650 °C for 1 hour, but sometimes temperatures as highas 700 °C. The image rightward[clarification needed] shows the area whereprocess annealing occurs.

Isothermalannealing

It is a process inwhich hypoeutectoid steel is heated above the upper critical temperature. Thistemperature is maintained for a time and then reduced to below the lowercritical temperature and is again maintained. It is then cooled to roomtemperature. This method eliminates any temperature gradient.

Normalizing

Carbon steel isheated to approximately 55 °C above Ac3 or Acm for 1 hour; this ensures thesteel completely transforms to austenite. The steel is then air-cooled, whichis a cooling rate of approximately 38 °C (100 °F) per minute. This results in afine pearlitic structure, and a more-uniform structure. Normalized steel has ahigher strength than annealed steel; it has a relatively high strength andhardness.

Quenching

Carbon steel withat least 0.4 wt% C is heated to normalizing temperatures and then rapidlycooled (quenched) in water, brine, or oil to the critical temperature. Thecritical temperature is dependent on the carbon content, but as a general ruleis lower as the carbon content increases. This results in a martensiticstructure; a form of steel that possesses a super-saturated carbon content in adeformed body-centered cubic (BCC) crystalline structure, properly termedbody-centered tetragonal (BCT), with much internal stress. Thus quenched steelis extremely hard but brittle, usually too brittle for practical purposes.These internal stresses may cause stress cracks on the surface. Quenched steelis approximately three times harder (four with more carbon) than normalizedsteel.

Martempering(marquenching)

Martempering isnot actually a tempering procedure, hence the term marquenching. It is a formof isothermal heat treatment applied after an initial quench, typically in amolten salt bath, at a temperature just above the "martensite starttemperature". At this temperature, residual stresses within the materialare relieved and some bainite may be formed from the retained austenite whichdid not have time to transform into anything else. In industry, this is aprocess used to control the ductility and hardness of a material. With longer marquenching,the ductility increases with a minimal loss in strength; the steel is held inthis solution until the inner and outer temperatures of the part equalize. Thenthe steel is cooled at a moderate speed to keep the temperature gradientminimal. Not only does this process reduce internal stresses and stress cracks,but it also increases the impact resistance.

Tempering

This is the mostcommon heat treatment encountered, because the final properties can beprecisely determined by the temperature and time of the tempering. Temperinginvolves reheating quenched steel to a temperature below the eutectoidtemperature then cooling. The elevated temperature allows very small amounts ofspheroidite to form, which restores ductility, but reduces hardness. Actualtemperatures and times are carefully chosen for each composition.

Austempering

The austemperingprocess is the same as martempering, except the quench is interrupted and thesteel is held in the molten salt bath at temperatures between 205 °C and 540°C, and then cooled at a moderate rate. The resulting steel, called bainite,produces an acicular microstructure in the steel that has great strength (butless than martensite), greater ductility, higher impact resistance, and lessdistortion than martensite steel. The disadvantage of austempering is it can beused only on a few steels, and it requires a special salt bath.

Case hardening

Main article:Case hardening

Case hardeningprocesses harden only the exterior of the steel part, creating a hard, wearresistant skin (the "case") but preserving a tough and ductileinterior. Carbon steels are not very hardenable meaning they can not behardened throughout thick sections. Alloy steels have a better hardenability,so they can be through-hardened and do not require case hardening. Thisproperty of carbon steel can be beneficial, because it gives the surface goodwear characteristics but leaves the core flexible and shock-absorbing.