Study On The Properties Of A 17-4PH Stainless Steel Used In A Nuclear Reactor

The type 17-4 precipitation hardening (17-4 PH) stainless steel is widely used asstructural materials for chemical and power plants, such as light water reactors(LWRs) and pressurized water reactors (PWRs) due to its high strength, highfracture toughness, good weldability and ease of machinability. These materials haveto service for a very long period of time during the life span of the power plants.Hence, understanding the microstructure evolution at the service temperature (about350℃) is very important.Based on a review of application, development and progress of 17-4PH stainlesssteel, the solid-state phase transition, hardening behavior and their influence onmechanical properties of the stainless steel subjected to long term intermediatetemperature aging treatment were researched systematically, and the salt bathingnitriding and plasma nitriding of 17-4PH stainless steel were researched, too, by thetheories of diffusion and phase transition in solid metal, and by means of variousanalytical techniques such as XRD, SEM, TEM, EPMA and DTA.When the 17-4PH stainless steel is tempered treated at 350-595℃for aboutcertain time after solution treating at 1040℃, the bulk hardness of the steel attains itspeak value, and then decreases at all time. When tempering temperature is lower, thepeak hardness will not attain in short aging time.TEM and XRD analysis shows that the fine spheroid-shape copper with the f.c.c. crystal structure precipitated after tempering treatment at 595℃for 4 hours,whereorientation relationship between martensite (b.c.c.) and copper (f.c.c.) can bedescribed as follows: (101)_M//(111)_Cu, [111]_M//[110]_Cu, which obeyed aNishiyama-Wassermann relationship and K-S relationship.Synchronously, the fiber-shape secondary carbide, M₂₃C₆, precipitated frommartensite matrix, where orientation relationship between martensite (b.c.c.) andfiber carbide (f.c.c.) can be described as follows: (101)_M//(511)_C, [111]_M//[149]_C.But, the appearance of lath martensite matrix is unchanged afer tempering at 595℃.Calculated by the thermodynamic, the nucleus driving force (â–³G_m) of precipitateofε-Cu in 17-4PH stainless steel is 13226.2 J/mol. The active energy of precipitationofε-Cu in 17-4PH stainless steel is 137.8KJ/mol by calculated from ageingprecipitation kinetics.The precipitationsε-Cu and M₂₃C₆, which both are coherent with martensitematrix, are responsible for strengthening of the alloy. In the following aging, theprecipitationε-Cu grows from the f.c.c structure to a critical dimension, theprecipitate loses the coherent relationship with matrix, which has not theprecipitation hardening effect. The precipitation of secondary carbide, M₂₃C₆,decreases the carbon content of matrix further, which lessens the strength of themartensite matrix, leading the bulk hardness decease.When 17-4PH stainless steel was subjected to long-term aging at 350℃, thespinodal decomposition occurred firstly at the grain boundary. The fine scalespinodal decomposition of martensite was Cr-richα′(bright image lamellae) andFe-richα(dark image lamellae), respectively, which have the alternated lamellaeimage of theα′andαphase perpendicular to the grain boundary. With prolongingaging time, the decomposition microstructure expanded from grain boundary tointerior, and its wavelength changed little. When the alloy is aged at 350℃for 9months, some reverted austenite is formed and theε-Cu precipitate ripened. Whenthe alloy is aged from 9 to 12 months, some bulk secondary carbides precipitated.Then the aging time is extended to 11,000hrs, a magnificent amount of reversed austenite transformed and the G-phase, a kind of intermetallic, precipitation occursnearby theε-Cu precipitate in the matrix. The orientation relationship between theG-phase andε-Cu (f.c.c.) can be described as follows: (111)G//(111)ε-Cu, [101]G//[101]ε-Cu. This relation suggests that a Cubic-Cubic relationship is obeyed.After the17-4PH stainless steel was subjected to long-term aging at 350℃,itsmechanical properties were changed with the aging time, for example the dynamicfracture toughness of the alloy was decreased in exponential decay form and theaging ebrittlement occurred. The fractography of fracture under different agingconditions is changed from the ductile fracture to brittle fracture with the extensionof isothermal aging time at 350℃. The tensile strength of the alloy increased with thegoing on aging. The higher aging temperature was, the bigger the increment oftensile strength was. But, the elongation and the contraction of area of the alloyreduced after aging.After 17-4PH stainless steel was subjected to long-term aging at 350℃, thecorrosion resistance lessened with the aging time increasing. The reason for this isthe various secondary phases precipitated during long term aging. The potentialdifference between the secondary phases and the matrix is responsible for thecorrosion resistance reduction.When 17-4PH stainless steel was subjected to the salt bathing nitriding, the mainphase of the nitrided layer was expanded martensite (α′),Fe_2-3(N,C),CrN,Fe₄N andFe₃O₄. The amount of Fe₃O₄ and CrN was increased with the treatment temperaturegoing up. The lattice constant of expanded martensite has the similar change. Theactivation energy of nitriding in this salt bath was 190.9kJ/mol.The depth of thenitrided layer was increased with the treatment temperature increasing. After thealloy nitriding at 580℃, the mass loss in the slide wear test was reduced from21.1mg for H1100 condition to 1.0mg. But, the corrosion resistance of the alloy in0.5MH₂SO₄+1%NaCl was reduced.When the 17-4PH stainless steel was subjected to the plasma nitriding, thenitiding layer with higher micro-hardness was attained. The wear resistance of the alloy after plasma nitriding compared with that for the salt bath nitiding. If theplasma nitriding temperature was above 400℃, the CrN was transformed in thenitrided layer.