| Martensitic precipitation hardening stainless steel represented by 17-4PH has high strength and toughness,excellent corrosion resistance,oxidation resistance,excellent formability and weldability.It is widely used in aviation,aerospace,chemistry,paper industry,nuclear industry,military industry,energy and other fields,such as aircraft engine(rotor,turbine blade,etc.),missile,chemical equipment,offshore platform,helicopter and nuclear waste bucket and so on.In recent years,the heat treatment process has been studied extensively,but the mechanism of strengthening and toughening has not been clearly clarified.In this paper,17-4PH forged rod was quenched to room temperature after being austenitized at 1040 ℃,and then air-cooled after aging at 480 ℃ for 0-5h.The microstructure evolution of quenching and aging process was observed and analyzed by optical microscopy(OM),hyper-field microscopy(HM),X-ray diffraction(XRD)and micro-hardness tester.The effect of dynamic aging precipitation process of epsilon-copper phase on electric resistivity was indirectly measured by resistor tester,and the wear resistance was tested by friction and wear tester.It is found that there was no retained austenite and reversed austenite in the quenched and aged 17-4PH steel.After quenching,there were two types of martensite: lathy and blocky.The hardness of lathy martensite was higher than that of blocky martensite.Although the hardness of two kinds of martensite increased synchronously with aging time,the hardness of lathy martensite was generally higher than that of blocky martensite.The precipitations obviously increased the hardness of martensite from 39 HRC to 49.5HRC;the friction coefficient of 17-4PH steel aged for 5h was much higher than that of quenched structure due to the increase of hardness(+10.8HRC);the electric resistivity decreased sharply due to the rapid precipitation of epsilon-copper phase during 0-0.5h aging process;and the electric resistance rebounded due to the coarsening and morphological transformation of epsilon-copper phase during 0.5-1.5h aging process.In the aging process of 0-5h,the hardness and wear resistance show the opposite trend with resistivity.According to the data of structure,hardness and resistivity,we can deduce the relevant rules of aging stage: In the first stage,the strength and hardness of 17-4PH increase rapidly to the peak value when aging time is 0-0.5h.In this stage,the rapid precipitation of epsiloncopper phase plays a dominant role in strengthening.For resistance,the rapid precipitation of epsilon-copper phase increases the free electrons in the matrix;In the second stage,the free electrons in the matrix increase when aging time is 0.5-1.5h.The strength and hardness show a retrogression process,which is due to the coarsening of the growth and morphology of the epsilon-copper phase,the interaction of dislocations,and the transformation of the matrix lattice from coherent to semi-coherent or non-coherent,resulting in tremendous lattice distortion,weakening the strengthening effect and increasing the electric resistivity.The third stage: 1.5 hours later,the slow precipitation of Nb(C,N)provides new power for strengthening.The precipitation of Nb(C,N)makes the content of solute atoms such as C,N and Nb in 17-4PH stainless steel matrix decrease,which causes the electric resistivity of the third stage of aging decrease.After heat treatment,the strength of the material changes in the same way as that of the hardness,contrary to that of the resistance.Plasticity and strength show the opposite trend,the same as the change of toughness,that is,the higher the strength of the material,the smaller the elongation,the lower the plasticity and toughness of the material. |
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