English
Menu

Surface treatment and tribology

Which statement about nitriding, such as gas nitriding, applied to a nitriding steel containing Al, Cr and Mo is correct?

  1. ANitrogen is diffused in the austenite region at about 900 °C and the part is then quenched to martensite, so it shows larger quench distortion than carburizing
  2. BPure iron and carbon steels without nitride-forming alloying elements let nitrogen diffuse deeper and faster, giving the thickest, hardest layers
  3. CNitrogen diffuses in at about 500–570 °C, in the ferrite region below A₁, and fine nitrides harden the surface without quenching, so distortion is small
Show answer and explanation

Answer: C. Nitrogen diffuses in at about 500–570 °C, in the ferrite region below A₁, and fine nitrides harden the surface without quenching, so distortion is small

Nitriding introduces nitrogen from the surface, for example by decomposing ammonia, and is carried out at about 500–570 °C, below the A₁ transformation temperature (727 °C). Nitrogen diffuses through ferrite and combines with Al, Cr, Mo, V and similar elements to form fine nitrides, and the surface reaches hardnesses above 1000 HV. With no phase transformation or quenching, dimensional change and distortion are small, so parts can be treated after finish machining. A compound layer of iron nitrides (the white layer) forms at the outermost surface, with a diffusion layer beneath it. In carbon steels without alloying elements, nitrogen may diffuse deeper, but few hard fine nitrides form, so the hardness does not rise much. 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.

Report an error

Practice this topic Back to topic

More Surface treatment and tribology questions

  1. Which statement correctly describes electroless nickel plating (Ni-P plating)?
  2. Cutting tools are coated with hard films such as TiN. Which is an appropriate way to choose the coating method for cemented carbide tools and for high-speed tool steel (HSS) tools?
  3. The hardness of a TiN film 2.0 µm thick is measured by indentation with a Vickers indenter. To avoid influence from the substrate, the empirical guideline of keeping the indentation depth to one tenth of the film thickness or less is used. The depth of a Vickers indent is about one seventh of its diagonal length. Which is closest to the largest allowable diagonal length?
  4. Which statement correctly describes the empirical law of friction (the Amontons–Coulomb law) that often holds for dry sliding friction between metals?
  5. A flat steel block (indentation hardness H = 2.0 GPa) rests on a plate of the same steel, and a normal load of 1.0 kN is applied over an apparent contact area of 10 cm². Assuming every contacting asperity deforms plastically and estimating the real area of contact as Ar = W/H, which is closest to its fraction of the apparent contact area?