A treatment in which low-carbon steel is held in a carbon-bearing atmosphere in the austenite region to raise the carbon content of the surface layer. Quenching then turns the surface into hard martensite.
Japanese name: 浸炭
Carburizing appears in 6 practice questions across 3 topics, including Crystal defects and diffusion, Surface treatment and tribology, Fracture mechanics and fatigue.
In parts such as gears and shafts, the surface must resist wear, high contact stress and bending fatigue, while the core needs toughness so that it does not crack under impact, and giving the whole part the same microstructure makes both hard to achieve. Carburizing uses low-carbon steel with around 0.2% carbon, raises the carbon content of the surface layer alone to around 0.8%, and then quenches it, so the surface becomes hard martensite while the core remains a tough low-carbon structure. Compressive residual stress also remains in the surface layer, raising fatigue strength as well. Case-hardening steels with added Cr or Mo, such as SCr420 and SCM420, are commonly used.
Carbon enters by diffusion from the surface, so the depth at which a given carbon content is reached is roughly proportional to √(Dt), and doubling the depth takes about four times as long. Taking the diffusion coefficient of carbon in austenite as D = 2.3×10⁻⁵exp(−148 kJ/mol ÷ RT) m²/s gives about 8×10⁻¹² m²/s at 927 °C. When a steel with 0.2% carbon is carburized for 4 hours with its surface held at 0.9%, the error-function solution estimates the depth at which the carbon content is 0.4% at about 0.5 mm. In reality the diffusion coefficient changes with carbon content, so this is a guide. Higher temperatures are faster but tend to coarsen the grains.
In the widely used gas carburizing, the carbon potential (a guide to the surface carbon content the atmosphere gives the steel) of an atmosphere consisting mainly of CO and H₂ made from hydrocarbons is controlled to set the surface carbon content. Because this atmosphere contains slightly oxidizing components, easily oxidized elements such as Si, Mn and Cr oxidize at grain boundaries in the surface layer; this intergranular oxidation leaves regions that harden poorly and lowers fatigue strength. Vacuum carburizing, in which hydrocarbons such as acetylene flow into a furnace at reduced pressure, causes no intergranular oxidation, allows short treatment at high temperature and carburizes the inside of holes more easily.
After carburizing and quenching, parts are usually tempered at a low temperature of about 150–200 °C to restore toughness without lowering the hardness much. Because the surface layer is rich in carbon, some austenite tends to be retained after quenching, and when dimensional change or insufficient hardness is a problem, a subzero treatment that cools the part below 0 °C is applied. Quality is checked by measuring the hardness profile on a cross section, and the depth at which the hardness falls to 550 HV is commonly taken as the effective case depth. Quench distortion is unavoidable, so gears requiring precision are finished by grinding the tooth flanks after carburizing and quenching.
Answer: C. About 4.8 hours
(0.60 − 0.20)/(1.00 − 0.20) = 0.50, so 1 − erf(z) = 0.50 and erf(z) = 0.50 give z = x/2√(Dt) = 0.477. Then √(Dt) = 0.50×10⁻³/(2 × 0.477) ≈ 5.24×10⁻⁴ m, Dt ≈ 2.75×10⁻⁷ m², and t = 2.75×10⁻⁷/(1.6×10⁻¹¹) ≈ 1.72×10⁴ s, about 4.8 hours. About 19 hours is the mistake of dropping the 2 in the denominator and setting x/√(Dt) = 0.477, which makes the time four times too long. About 7.9 hours ignores the initial carbon concentration C₀ and uses 0.60/1.00, giving erf(z) = 0.40 and z = 0.371. Because the time is proportional to the square of the depth, how the factors are handled has a large effect.
Answer: A. Carbon is diffused into the surface in the austenite region (about 900–950 °C) before quenching, giving hard martensite at the surface and a tough core
Carburizing is done in the austenite region (about 900–950 °C), where carbon solubility is large (about 1.2–1.4% in the 900 °C range) and diffusion is fast. When the surface carbon content is raised to around 0.8% and the part is quenched, the surface becomes hard martensite and the core a tough low-carbon structure, combining wear resistance with impact resistance. The carbon-rich surface has a lower martensite start temperature and transforms later than the core, so its volume expansion is constrained by the core and compressive residual stress remains in the surface, which also raises fatigue strength. The solubility limit of carbon in ferrite is at most about 0.02%, and hardening without quenching in the low-temperature ferrite region is a feature of nitriding. Carburizing and quenching requires measures against quench distortion.
Carburizing refers only to the treatment that adds carbon to the surface layer, and by itself it does not harden the steel. The whole sequence of quenching afterward to turn the surface into martensite, and then tempering, is called carburize hardening (carburizing and quenching). In practice, carburizing often refers to the whole sequence including quenching.
Carburizing and quenching can give a deep case of about 0.5–2 mm and suits heavily loaded gears and similar parts, but quenching causes distortion. Nitriding gives a thinner case but hardens the steel without quenching at temperatures below the transformation point, with little distortion, so it suits parts that need dimensional accuracy.
They are covered with copper plating or a stop-off paint so that carbon cannot enter. This is used to leave areas unhardened, such as threads or areas to be machined later. Another method is to cut those areas away after carburizing but before quenching.
Carburizing is done in the austenite region (about 900–950 °C), where carbon solubility is large (about 1.2–1.4% in the 900 °C range) and diffusion is fast.
A gear made of low-carbon steel (about 0.2%C) is carburized…Because D changes exponentially with temperature, raising the carburizing temperature by just a few tens of degrees can shorten the treatment time considerably.
The diffusion coefficient of carbon in γ-iron follows the A…The diffusion-distance estimate x ≈ √(Dt) is used in many situations, including carburizing and dopant diffusion.
In a diffusion treatment at a fixed temperature, by what fa…Carburizing, nitriding and induction hardening also harden the surface layer and introduce compressive residual stress, so they raise fatigue strength.
Which surface treatment can be expected to improve the fati…On the other hand, the case is about 0.1–0.5 mm, thinner than with carburizing, and treatment often takes several tens of hours, which are drawbacks.
Which statement about nitriding, such as gas nitriding, app…Nitriding Diffusion coefficient Martensite Abrasive wear Adhesive wear Archard wear equation Electroless nickel plating Real area of contact Stribeck curve