Papers by Author: Hans Berns

Abstract: Powder metallurgy represents a good alternative to a conventional casting process to produce wear resistant materials. MMC (metal matrix composite) of a hardenable steel matrix and hard phases compacted by hot isostatic pressing (HIP) are highly wear resistant but high in price. In the present study liquid phase sintering was preferred to HIP and expensive hard phases as tungsten carbides were replaced by ferrotitanium particles (FeTi) to reduce costs. A mixture of gas atomized hot work steel powder of < 150 µm in size, hypereutectic FeBC powder of < 36 µm in size as liquid phase donor (LPD) and ferrotitanium particles (100-150 µm in size) with about 70 mass% of Ti was sintered in vacuum to give a wear resistant MMC of full density. However, the elements B and C from LPD diffused to the steel powder before the liquid phase appeared. Therefore these two powders merged into a near-eutectic or hypoeutectic constitution. The LPD was diluted by the steel. During sintering the ferrotitanium particles are transformed in situ into hard phases (in situ HP) with a Fe-rich core surrounded by a hard TiC case to withstand abrasive wear. Numerous investigations by LOM, SEM with EDX, WDX and DTA were realized step by step for a deeper understanding of what happens in the initial three-component mixture during liquid phase sintering and how the in situ MMC forms. The resistance to wear was measured by pin-on-plate tests against abrasive paper of different hardness and mesh size and compared with HIP-MMC and SLPS-MMC.

Abstract: Joint alloying of 0.85 to 1.1 mass% C + N raises the strength and cold work hardening of steels with 18 to 19 mass% Cr and Mn each and allows to produce them at atmospheric pressure. A yield strength of 600 MPa is combined with a true fracture stress of almost 2500 MPa and ≈ 70 % elongation. However, there is a risk of carbide/nitride precipitation during quenching of thicker cross sections after solution annealing. The addition of Mo and Cu affects the corrosion resistance as well as the precipitation. Submersion test and current density/potential tests in several aqueous solutions characterize the corrosion behaviour. Tests on intercrystalline corrosion are used to detect the precipitation as a function of quenching rate. It is shown that the C/N ratio is of key importance in improving the properties.

Abstract: Measurements of conduction electron spin resonance (CESR) in steel allow to separate the contributions from free electrons which provide the metallic character of interatomic bonds and from localized electrons involved in the covalent bonds. The data of the CESR study carried out on austenitic CrMn steels alloyed with carbon, nitrogen or carbon+nitrogen are presented. It is shown that, in contrast to carbon, nitrogen enhances the metallic character of atomic interactions with a maximum of the concentration of free electrons at some critical content of nitrogen (about 2 at.%). The combined alloying with carbon+nitrogen leads to two effects: (i) a larger concentration of free electrons and (ii) a shift of the critical content of interstitials towards higher values. The experimental data are supported by theoretical ab initio calculations of the electron properties of austenitic CrMn steels alloyed with carbon, nitrogen or carbon+nitrogen. Using the full-potentialfull- electron-linearized-augmented-plane-wave (FLAPW) method, the total energy per primitive crystal cell, the density of the electron states (DOS) and the distribution of the electron density over the crystal lattice were calculated by means of the computational program WIEN-2k. The total electron energy decreases due to alloying in the sequence of carbon→nitrogen→carbon+nitrogen, which suggests a corresponding increase in the thermodynamic stability of the austenite. The obtained results of the theoretical and experimental studies of the electron structure were used for the development of super-high–strength stainless austenitic steels.

Abstract: The solubility of nitrogen is high in stainless austenite of steels with 18 mass% of Cr and Mn each, but low in the melt. Carbon reveals the opposite behaviour. Instead of producing high nitrogen steels by pressure metallurgy (PHNS), about 1 mass% of C+N is dissolved in the melt at ambient pressure. The new cost-effective C+N steel reaches a yield strength of 600 MPa, a true fracture strength above 2500 MPa and an elongation above 70 %. Conduction electron spin resonance revealed a high concentration of free electrons. Thus, the ductile metallic character of the C+N steel is enhanced, explaining the high product of strength and toughness.