R.D. Mercado-Solis

Development of Mathematical Model for Control Wear in Backup Roll for Hot Strip Mill

The precision of strip flatness depends on several factors; wear of rolls is one of the main variables that have influence on the surface quality of the strip. The wear of the rolls represents a complex friction condition, sometimes the wear in the backup roll is not analyzed because the strip is not in contact with the backup roll; however, after several campaigns of rolling, the wear in the backup roll becomes dangerous because the pressure distribution is not uniform. Investigation of mechanism of the surface deterioration of the backup roll for the hot strip rolling is very important for the development of the automatic strip shape control system used in hot strip mills. A mathematical model is developed considering the Hertzian pressure distribution between two cylinders with parallel axes. It is used in real time for calculating the wear in the backup roll and in this manner take decisions for preventing finished product reworking or damage of equipments, which result in accidents caused by excessive wear in the backup rolls.

High temperature oxidation of silicon and copper-silicon containing steels

Abstract The isothermal oxidation behaviour of silicon and copper containing steels was studied under laboratory dry air conditions at temperatures from 950 to 1180°C. The oxidation kinetics was determined by gravimetric means and followed by parabolic relationships at all temperatures for both steels. Characterisation of the oxide scales comprised X-ray diffraction, light optical microscopy and SEM including EDX analysis. It was found that the oxide microstructure depended on the silicon and copper contents in the alloys. For the copper containing steel oxidised at 950°C, enrichment of this element was not appreciated at the metal/oxide interface or across the oxide scale, a situation that changed at 1100°C where copper enrichment was localised next to the steel substrate. The specimens oxidised at 1180°C revealed that copper enrichment was located at the surface of the specimens. The effect of the alloying elements on the oxidation rate and the microstructure of the oxides formed on the steels are discussed.

The Design of a Laboratory Test Machine to Simulate Thermal Fatigue in Hot-Working Tool Steels

A laboratory-scale thermal fatigue simulator has been designed, constructed, and commissioned by the authors for studying thermal fatigue of hot-working tool steels by means of rapid alternated heating and cooling. The basic design features, construction characteristics, and test capabilities of the thermal fatigue simulator are presented in this paper. Thermal fatigue simulations were run on hardened and tempered AISI H13 hot-working tool steel specimens in time-control with heating and cooling times of 15 and 10 s, respectively, and a total number of 500 and 2000 thermal fatigue cycles. The experimental results have demonstrated that the simulator is capable of producing thermal fatigue cracks with the same characteristics of those seen in real industrial hot-working tools. Based on their size and the extent of propagation, a clear distinction between primary, secondary, and craze cracks could be established at the failed surfaces. Additionally, a thermo-mechanical finite element model of the first 10 thermal fatigue cycles was developed to compute the transient temperatures, stresses, and strains distributions within the test specimen during thermal cycling. Based on the model results, the low cycle fatigue life was estimated using the Coffin–Manson equation, which relates the number of cycles to crack initiation to the plastic strain range per cycle. The experimentally obtained fatigue lives were appreciably shorter than the calculated ones, arguably due to surface roughness and oxidation effects.

Effect of Different Quench Media on the Microstructure and Mechanical Properties of Large-Scale Low Alloy Steel Forgings

Large-scale forging segments with two different section thicknesses (100 and 250 mm) were separately subjected to water, polymer solution and vegetable oil quenching and then tempered to evaluate the influence of cooling rate on microstructure and mechanical properties under industrial conditions. Regardless of quenching media, the fastest cooling rates are obtained in the thin (100 mm) sections of the ring segments. For the two cross sections, water and vegetable oil generated the faster and slower cooling times, whilst an intermediate cooling timebetween that of water and oil was achieved with polymer solution. Slightly enhanced mechanical properties in the thin sections are associated to the presence of fine mixtures of tempered martensite and tempered bainite (lower type morphology) compared to the thick section microstructure mostly composed of coarse tempered bainite with granular and lower type morphologies and small amounts of tempered martensite. The results obtained in this study also suggest the possibility of using vegetable oil as an alternative quenching media for large-scale forgings with high-specification requirements.

Partial Decarburization and Intensive Quenching to Increase Fatigue Limit of Quenched Small Parts

The 5160 spring steel is mainly used to endure fatigue and since its Mf is below room temperature, retained austenite is usually present after conventional quenching, which is detrimental from a fatigue point of view. To reduce the amount of retained austenite, a partial decarburization was promoted (to increase the Ms and Mf, respectively) by austenitizing the steel at different temperatures and times in air, prior to performing an interrupted quenching in an accelerated media (brine) instead of the conventional oil quenching. The same quenching conditions with partial decarburization and without decarburization are compared. In addition, conventional oil quenching was performed as reference. Results showed that with a controlled partial decarburization, it is possible to increase the fatigue limit compared to the conditions without decarburization (both interrupted quenching and oil quenching). Fatigue experiments were carried out in a rotating bending fatigue type tester. Cooling curves, decarburization profiles, fraction of martensite plots, and fatigue data are discussed.