The Study On Formation Mechanism Of Borided Layer Of H13Hot Work Die Steel With Pack Boriding Treatment At Low Temperature

Boriding treatment is one of the key technologies for improving the service life ofH13hot work die steel. Traditional boriding treatment is usually carried out underhigh temperatures, which is an energy-wasteful method. At the same time, traditionalboriding treatment can easily lead to the severe deformation of workpiece. Therefore,boriding treatment at low temperature is the top research aspects in the field ofboriding. As surface nanocrystallization and catalysis of rare earth can decrease thetemperature of chemical heat treatment, the study on boriding treatment at lowtemperature that is base on the combination of boriding technology and twotechnologies as mentioned above possesses important academic significance andengineering application significance. In this dissertation, cyclic shot peeningtechnology is invented, which can fabricate a nanostructured surface layer on H13steel with heat treatment. Meantime, pack boronizing agent is invented, which canprepare borided payer on H13steel with polished surface (P sample) andnanostructured surface (CSPT sample) at low temperature. The microstructure andcomposition, phase composition, hardness gradient, dynamic erosion resistance andthermal fatigue property of borided payer were systematically investigated by X-raydiffraction (XRD), scanning electron microscopy (SEM), energy dispersivespectroscope (EDS), transmission electron microscopy (TEM), glow discharge opticalemission spectrometry (GDOS), nanoindentation, experimental set up used fordynamic erosion test, experimental set up used for thermal fatigue test and so on.Meanwhile, the formation mechanism of borided layer fabricated by boridingtreatment at low temperature was studied deeply. The catalytic mechanism of rareearth, the thermodynamic condition and growth kinetics model of forming boridedlayer were discussed. The main results are as follows:1. After CSPT was carried out for surface layer of H13, the hardness, residual stress,dislocation density, microstrain, storage energy of deformation of it are7.4GPa,-543MPa,1.2×1015m-2,0.25%and4.2×106J·m-3, respectively. The mean grain size is 25nm in the top surface layer (of about2μm thickness). The nanocrystal does notgrow up obviously below600℃. The nanocrystallization mechanism is dominated bythe combinatorial movement of dislocation glide, tangles and dislocation walls. Whenthe strain is small, dislocation cells form in in original ferrite grains and transform intosubgrains at low strains. With the strain increasing, the lamellar structures thatintersect each other form and its thickness decreases continuously, and approximateequiaxed grain with submicron size could form. Finally, the equiaxed nanocrystallitesin the top surface layer can be fabricated, due to the combined action of the largestrain with a high strain rate and the multidirectional force.2. Pack boronizing agent that can be used at low temperature is50%B4C+10%NaBF4+5%NH4BF4+5%NH4HCO3+10%CeCl3+4%C+16%SiC. Withpack boriding at580℃for10h, the thickness of borided layer of P sample and CSPTsample are7.19and9.23μm, respectively. The hardnesses of them are19GPa and23GPa, respectively. C and Si did not diffuse into transition zone in the process ofboriding treatment. Therefore, there is no low hardness zone beneath borided layer.The phase compositions of borided layer are FeB and Fe2B. There are stacking faultstructures in the two borides. There are also twin structures in them, which possesspartial dislocations with high density in twin boundaries.3. The rare earth of Ce can accelerate the adsorption of active boron on the surfaceof sample. The Ce nonuniformly distribute in the surface layer, which enrich in thegrain boundaries. Meanwhile, Ce can trace dissolve in grain. When the rare earthdiffuses into the surface layer of substrate, the grain recovery, recrystallization andgrowth of-Fe phase can be inhibited. At the same time, the rare earth that dissolvesin grain could coexist with lattice vacancy and the vacancy concentration of crystallattice can be increased. Therefore, the activation energy for active boron to diffuseinto the substrate can be decreased and the velocity of diffusion mobility of it whichdiffuses into the surface layer of substrate could be increased significantly4. When the temperature is580℃, the diffusion coefficients of boron atom in-Fephase of P sample and CSPT sample are5.41015m2/sand9.21015m2/s, respectively. The free energy changed of forming FeB for P sample and CSPT sample are-157.8kJ/mol and-179.9kJ/mol; respectively; The free energy changes of forming Fe2B for P sample and CSPT sample are-181.9kJ/mol and-204.0kJ/mol, respectively; The diffusion coefficients of boron atom in the borides for P sample are DFcB=3.80×10-14m3/s and DFe2B=1.90×10-14m2/s, respectively; The diffusion coefficients of boron atom in the borides for CSPT sample are DFeB=6.82×10-14m2/s and DFe2B=3.83×10-14m2/s, respectively. The results as mentioned above show that surface nanocrystallization can significantly accelerate the rate of diffusion and migration of the boron atoms in boriding treatment at low temperature.5. When the temperature is at580-680℃, the apparent diffusion activation energy for forming borided layers of P sample and CSPT sample with pack boriding at low temperature are96.41kJ/mol and71.87kJ/mol, respectively. When compared to apparent diffusion activation energy of P sample, the corresponding value of CSPT sample decrease by25kJ/mol, which demonstrates that surface nanocrystallization can improve the growth efficiency of borides. The kinetic equation of growth for P sample and CSPT sample which reflect the relationship between the thickness of borided layer and boriding temperature and time are as follows:6. Owing to the corrosion resistance of borided layer, it can effectively prevent the substrate directly contacting with molten aluminum alloy. In consequence, the chemical reaction of forming intermetallic compound on the surface of sample cannot occur easily and the dynamic erosion resistance of borided sample can be improved remarkably. At the sample time, the borided layer with excellent oxidation resistance and mechanical strength at elevated temperatures could effectively delay the thermal fatigue cracks initiation and impede their propagation. Therefore, the thermal fatigueproperty of H13steel with boriding treatment at low temperature can be improvedsignificantly.