Shrinkage and Cracking Sensitivity of Cement Mortar Containing Fly Ash, Granulated Blast-furnace Slag and Silica Fume

A laboratory study was undertaken to investigate drying shrinkage and cracking sensitivity subjected to restrained shrinkage of mortar containing fly ash (FA), granulated blast-furnace slag (GBFS) and silica fume (SF). Six mortar mixtures including control Portland cement (PC) and FA, GBFS and SF mortar mixtures were prepared. FA replaced the cement on mass basis at the replacement ratios of 20% and 35%, GBFS replaced the cement at the replacement ratios of 40%, SF replaced the cement at the replacement ratios of 8% and the blended mixtures with 20% FA, 20% GBFS and 8% SF. Water-cementitious materials ratio and sand-cementitious materials ratio were 0.4 and 2.0 for all mixtures, respectively. The mixtures were cured at 65% relative humidity and 20℃. The drying shrinkage value, initial cracking time and cracking width of the mortar samples were measured. The results show that all the mortar mixture containing FA exhibited the decrease of drying shrinkage. Moreover, initial cracking time was markedly delayed, and the crack width of the initial crack was reduced. However, the incorporations of various ratios of GBFS and SF led to an increase of drying shrinkage, initial cracking time and cracking width as compared to control mixture.

Study of Cracking Mechanism and Wear Resistance in Laser Cladding Coating of Ni-based Alloy

Nickel-based alloy coatings were widely used for the remanufacturing of dies and moulds by laser cladding,but the crack sensitivity would be increase due to the higher strength and hardness,which reduced the wear resistance of Ni-based alloys.In this paper,Ni-based coatings with the addition of a plastic phase(an austenitic stainless net)were prepared using laser cladding technology,and the CeO_(2)was added in cladding layers.The cracking mechanism,microhardness,microstructure,phase composition,and wear properties were investigated.The relationship between thermal stress and the elastic and plastic fracture had been developed from the standpoint of fracture mechan-ics and thermal elastic fracture mechanics.The fracture criterion of the nickel-based coating was obtained,and the study has shown that the crack sensitivity could be reduced by decreasing the thermal expansion coefficientΔα.Thus,a new method was proposed,which the stainless steel nets were prefabricated on the substrate.It was found that the number of cracks reduced significantly with the addition of stainless steel net.When the stainless steel net with 14 mesh was added in Ni-based coatings,the average microhardness of nickel composite coating was 565 HV_(0.2),which was 2.6 times higher than that of the 45 steel substrate.Although the rare earth oxide 4 wt.%CeO_(2)and stainless steel net were added in the Ni-based coating reducing the microhardness(the average microhardness is 425 HV_(0.2)),the wear resistance of it improved substantially.The wear volume of Ni-based composite coating was 0.56×10^(−5) mm^(3)·N^(−1)·m^(−1),which was 85.1%lower than that of 45 steel.The experiment results have shown that the Nickel-based composite coating is equipped with low crack sensitivity and high abrasive resistance with austenitic stainless net and the rare earth oxide 4 wt.%CeO2.This research offers an efficient solution to produce components with low crack susceptibility and high wear-resistance coatings fabricated by laser cladding.

Solidification crack characteristics and sensitivity of peritectic steel

The characteristics and sensitivity of solidification cracks in peritectic steels were investigated using directional solidifi-cation technology.Interdendritic cracks were observed in both hypoperitectic steels(12CrlMoV,15CrMo)and hyper-peritectic steel(20CrMo)during solidification at growth velocities of 15,50,and 80 pm/s.At the dendritic boundaries,sulphide precipitates were found,promoting crack formation.Based on the statistical analysis of interdendritic cracks in peritectic steels,the area ratio(RA)of interdendritic cracks in a directional solidification structure was proposed to evaluate the crack sensitivity of peritectic steels.Furthermore,the crack sensitivities of peritectic steels(12CrlMoV,15CrMo,and 20CrMo)were tested,evaluated,and compared with the surface crack rates of three types of steels produced from a steel plant.The results demonstrated that RA was in good agreement with that of the steel plant,and the crack sensitivity of 12CrlMoV steel was the strongest,followed by that of 15CrMo and 20CrMo steels.Thus,RA can be used to evaluate the crack sensitivity of peritectic steel.

Prediction of Hot Ductility of Low-Carbon Steels Based on BP Network

The purpose of the research is to obtain an effective method to predict the hot ductility of low-carbon steels, which will be a reference to evaluate the crack sensitivity of steels. Several sub-networks modeled from BP network were constructed for different temperature use, and the measured reduction of area (A(R)) of 12 kinds of low-carbon steels under the temperature of 600 to 1000 degreesC were processed as training samples. The result of software simulation shows that the model established is relatively effective for predicting the hot ductility of steels.

Physical Simulation to Determine High TemperatureMechanical Behavior of Continuously Cast Steels

Hot ductility and strength of Continuous casting(CC) steels at elevated temperatures from CC processeswere studied by physical simulation. The method is the hot ductility test. The design of test parameters and datainterpretation are discussed. The results show that the bulging of CC steel slabs which is caused by the mechanismof creep has great influence on the formation of central segregation and internal cracks. Creep tests including staticcreep tests and dynamic creep ones were performed at 1200 and 1300℃. Effects of strain rate and temperature onhot ductility are also discussed and a simple model is presented to explain the interaction between hardening and softening.