Metallurgy of Highly Wear-Resistant Indefinite-Chill Work Roll Materials for Hot Rolling Mills

Indefinite-chill materials are used as shell materials for cast work rolls for surface-critical applications in hot rolling mills. Besides a smooth surface quality, a low sticking tendency and low sensitivity against incidents in the rolling mill, the work rolls need the highest wear resistance possible. The microstructure of the indefinite-chill material consists of various carbides (cementite up to 40 area-%) and up to 5 area-% of graphite embedded in tempered martensite. To increase the wear resistance of this material group, the comparably soft cementite has to be replaced by more wear resistant carbides such as MC, M2C or M6C. This can be achieved by increasing the amount of carbide forming elements such as Nb, V, Mo, W or Cr. Nevertheless it is important to maintain a certain amount of graphite in the microstructure to avoid sticking to the rolled material and to lower the sensitivity against mill incidents. It is well known that high amounts of carbide forming elements limit the graphite precipitation and therefore a sophisticated alloying concept is required for this material type. Not only the effects of matrix elements such as Si, Mn, Ni and Co but also the effects of Cr, Mo, W, Nb and V were studied in an intensive research project. This work gives an insight in the results of the project based on the example of the effects of Si and Cr on the phase amounts and the composition of the cementite phase.

Effect of pearlite on stress corrosion cracking of carbon steel in fuel-grade ethanol

The selective dissolution of ferrite phase from the pearlite was studied in fuel-grade ethanol (FGE) to understand how it affects the stress corrosion cracking (SCC) mechanism of carbon steel in FGE. It was shown that microgalvanic coupling occurs between ferrite and cementite phases of the pearlite, leading to localized corrosion, which affects the SCC mechanism. The intergranular SCC mechanism stops at the pearlite, and the selective dissolution promotes the transgranular SCC mechanism. Cathodic polarization curves were measured for pure iron and cementite exposed to various FGE conditions. According to the results, cementite phase is, in most cases, a more favorable cathode in FGE.

Cementite Dissolution in Cold Drawn Pearlitic Steel Wires: Role of Dislocations

Despite numerous investigations in the past, mechanism of cementite dissolution has still remained a matter of debate. The present work investigates cementite dissolution during cold wire drawing of pearlitic steel (~ 0.8wt% carbon) at medium drawing strain (up to true strain 1.4) and the role of dislocations in the ferrite matrix on the dissolution process. Quantitative phase analysis using x-ray diffraction (XRD) confirms more than 50% dissolution of cementite phase at drawing strain ~ 1.4. Detail analysis of the broadening of ferrite diffraction lines confirms presence of strain anisotropy in ferrite due to high density of dislocations (~ 1015m-2) at drawing strain 1.4. The results of the analysis shows that the screw dislocations near the ferrite-cementite interface are predominantly responsible for pulling the carbon atoms out of the cementite phase leading to its dissolution.

Synchrotron Diffraction Study of the Cementite Phase in Cold Drawn Pearlitic Steel Wires

Energy dispersive synchrotron diffraction has been carried out on cold drawn pearlitic steel wires. In this paper the observed cementite peaks are analysed. For a broad range of true drawing strains sin²(Ψ) curves have been measured. The residual stress in the cementite is found to saturate after reaching a maximum at a strain of about 1.6. No indication of significant texture development in the cementite could be observed. An explanation is given in terms of possible physical mechanisms. Peak broadening was observed at the early stages of deformation.

A Thermal and Acid Treatment for Carbon Extraction from Cast Iron and Its Application to Ams Dating of Cast Iron Objects from Ancient Korea

A method of thermal and acid treatments was developed at the Archaeo-metallurgy Laboratory of Hongik University in Korea to extract carbon from cast iron, and carbon objects thus prepared from cast iron artifacts of ancient Korea were dated at the University of Arizona's AMS Facility. The thermal treatments consist of heating a specimen to ∼1000 °C in a controlled environment with reduced oxygen potential, then cooling it rapidly to room temperature. The heating causes the cementite phase in white cast iron to be graphitized and the quenching suppresses pearlite formation. The specimen then consists of flakes of graphite embedded in a matrix of martensite. The next stage of the treatment is to dissolve the martensite matrix in a solution of nitric and hydrochloric acids to release the graphite as a powder. This material is then cleaned, dried, and pressed into target holders for accelerator mass spectrometry (AMS) analysis. The method was applied to a collection of artifacts from the Korean Three Kingdoms period (about AD 300–668) and the AMS results were compared with chronological estimates from other means.