Mechanical properties of nickel-chromic austenitic alloy 47ХНМ have been studied. It was shown that at increase of heating time for hardening the deforming pressures fall, plasticity grows that is connected with dissolution of excess a-phase. In order to obtain high strength properties with minimum level of elastic imperfections the thermal processing of alloy should be carried out in temperature range 650...750 °С, and time of ageing should be from 8 to 20 h depending on temperature of agein

Erbolatuly, D.

Isaeva, Zh. M.

Mukazhanov, E. B.

Skakov, M. K.

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Nickelchromic alloy 47ХНМ possesses high corrosion resistance, low temperature coefficient of modulusof elasticity, nonmagnetic property, low elastic hysteresis and elastic aftereffect, high fatigue strength and usedin industry not only as spring material [1] but also as anelement of construction of nuclear and thermonuclearreactors [2].Earlier in [3] influence of hardening temperature,curing time for hardening and cooling rate for phasestructural state of alloy 47ХНМ was carefully examined.The aim stated in this work consisted not only in studying structuralphase transformations and propertiesof alloy 47ХНМ but also in the fact that to show concrete ways of implementing the obtained results for increasing technological plasticity and strength propertiesof this material.Experiment The object of investigation is alloy 47ХНМ of industrial production and standard chemical composition(47 % – Cr, 5 % – Mo, res. – Ni).Mechanical tests of samples at room temperature onuniaxial tension were carried out at the device of the type «POLYANI» by standard technique in accordancewith SS 149784. By stressstrain diagrams yield pointsand ultimate strength were calculated and sample specific elongation after breaking was determined as well.Sample structuralphase state was studied at optical(NEOPHOT21) and electron (EM125К) microscopes. Sections for metallographic investigations were polished and etched by electrolytic method in 10 % aceticchloric electrolyte. Samples for electron microscopyin the form of discs were prepared by the method ofspray electropolishing as well as by the method of foilthinning.Results and their discussion After hardening in temperature range 900...1300 °Сstructure of alloy 47ХНМ is twophase consisting of grains of γmatrix and particles of αphase on the basis ofCr, having bcc lattice (Fig. 1, а). Increasing curing timefor hardening the dissolution of strengthening αphase,growth of matrix grains, increase of alloy anisomerousness and change of grain boundary character were stated.Growth of grain in alloy 47ХНМ is inhibited very muchdue to the presence of excess αphase which deceleratesboundary migration at collective recrystallization.Dependence of plasticity and resistance to deformation of alloy 47ХНМ on hardening temperature is givenin Fig. 2. It is seen that at hardening temperature growththe alloy plasticity increases and deforming stresses (σ0,1is the yield point, σ1 is the stress at residual deformationin 1 %, σВ is the ultimate strength) steadily decreases.Increase of plasticity at hardening temperature increaseis conditioned not only by dissolution but also by theprocesses of coalescence and spheroidezation of the excess αphase.Analysis of dependence of mechanical properties oncuring time at 1300 and 1225 °С shows that at increaseof heating duration deforming stresses decrease and plasticity grows. Decrease of resistance to deformation aswell as increase of alloy plasticity is conditioned by dissolution of excess αphase. The process of dissolutionbegins with disperse particles and at curing time increase larger particles are dissolved enriching matrix solidsolution with alloying component. Plasticity grows atincreasing homogenization time at 1300 °С up to 30 mininclusive and after that the curve comes to saturation.The character of changing mechanical properties depending on curing time at 1250 and 1225 °С obeys to thesame law that at 1300 °С but occurs at lower intensity.Bulletin of the Томsк Pоlytеchnic University. 2007. V. 311. № 2114UDC 538.9:621.785.6FEATURES OF CHANGE OF MECHANICAL PROPERTIES OF PRECIPITATION HARDENING ALLOY47ХНМ AT HARDENING AND AGEINGE.B. Mukazhanov, M.K. Skakov, Zh.M. Isaeva, D. ErbolatulyS. Amanzholov’s EasternKazakhstan state university, UstKamenogorsk, KazakhstanMechanical properties of nickelchromic austenitic alloy 47ХНМ have been studied. It was shown that at increase of heating time forhardening the deforming pressures fall, plasticity grows that is connected with dissolution of excess αphase. In order to obtain highstrength properties with minimum level of elastic imperfections the thermal processing of alloy should be carried out in temperature range 650...750 °С, and time of ageing should be from 8 to 20 h depending on temperature of ageing.a b c dFig. 1. Microstructure of alloy 47ХНМ: a) hardening from 1250 °С, 1 min, ×8500; b) ageing at 600 °С, 10 h, ×6500; c) ageing at 700°С, 10 h, ×8500; d) ageing at 1000 °С, 1 h, ×8500Fig. 2. Dependence of plasticity and resistance to deformationof alloy 47ХНМ on hardening temperatureAfter hardening from 1200 °С the value of yield point and ultimate strength of samples are higher morethan 10 kg/mm2 in comparison with temperature range1300...1250 °С. Character of changing plasticity at hardening temperature 1200 °С changes sharply instead ofexpected growth of plasticity at heating time increase itsfall is observed. The reason of such phenomenon is notdetermined although structural investigations of samplefracture mode in crosssection show the presence of socalled structure «fishscale» fracture. It is appropriate toconsider that «fishscale» structure is not the feature ofheat treatment flaw however, it may well be so that itmay be one of the reasons of decreasing plasticity andviscosity of alloy.Rather wide spread of values of plasticity andstrength values at testing sample hardened from 1200 °Сshould be indicated. Obviously, spread of values is influenced not only by structure inhomogeneity but also partial flashing of a particle of αphase being on grainboundary as well as the presence of undissolving particles which are stress concentrators. All these reasons result in microcrack formation decreasing alloy plasticityand viscosity.As precipitation hardening alloys are mainly usedafter treatment including hardening and ageing then theinvestigations of ageing influence on structure andmechanical properties of alloy 47ХНМ were of interest.At ageing at 600 °С of hardened alloy 47ХНМ at ageing time increase the strength properties insignificantlygrow (Fig. 3). Comparing the data of structural investigations (Fig. 1, b) at change of strength properties(Fig. 3) some conclusions may be made and namely:contribution into alloy hardening is, obviously, conditioned only by decay inside the excess phase but as thevolume fraction of this phase is low (5...10 %) and sizeof extractions inside these particles amounts from 15 to40 A then increase of strengthening is low. There is nostructural change inside the matrix in comparison withthe hardened material (up to 10 h of ageing (Fig. 1, b)and therefore, its contribution into alloy hardening maybe neglected. Certain decrease of plasticity occurs, probably, owing to formation of segregation of alloyingcomponents on grain boundaries.Fig. 3. Mechanical properties of alloy depending on ageing timeat 600 °С hardened preliminary from 1250 °С, 2 minThe data on kinetics of alloy hardening at 700 °Сhardened preliminary at 1250 °С are given in Fig. 4. Atthe initial stages of ageing the increase of resistance tosmall plastic deformation is already noted. Its valuegrows sharply at further increase of ageing time. The noted increase of hardening responds to decay stage as aresult of which the intermittent decay with the extraction of noncoherent αphase is intensively developed(Fig. 1, c).Electronmicroscopic and metallographic investigations of structure showed that the intermittent decay begins from grain boundary and finishes in 5...10 hours after beginning of ageing (Fig. 1, c), and decay volumefraction amounts to 75...85 % that responds to maximalhardening. The nature of hardening at this ageing temperature is conditioned by retardation of dislocation,intermittently extracted particles of αphase.At increase of ageing duration alloy plasticity fallssteadily and decreases to 5 % for 10 h of ageing while itamounted to 25 % for hardened alloy. Such fall is wellexplained if sharp increase of strength is taken into account. Besides, it should be noted that sensitive elements work in elastic domain with very small residualdeformation and therefore, such reserve of plasticity isenough for safe performance of devices.Fig. 4. Mechanical properties of alloy 47ХНМ depending on ageing time 700 °С hardened preliminary from 1250 °С,2 min?1?0,1???0,2??, ??/??2 ?, %t, ?? ??/??2 ?, %????1?0,1???1?0,1 ??, ??/??2 ?, %Mathematics and mechanics. Physics115At further increase of ageing temperature the changein the character of dependence of alloy strength properties on ageing duration is observed. The data on the influence of ageing time at 800 °С on microflow resistance, deforming stresses and plasticity of alloy 47ХНМ are givenin Fig. 4. First of all it should be noted that at initial stagesof ageing the deforming stresses and especially yield pointhave the value not worse than properties of alloy aged at700 °С. Therefore, such mode may be recommended atmanufacturing resilient sensitive elements as in this casetime of thermal treatment decreases considerably.Increasing ageing time the yield point and other deforming stresses including ultimate strength decreasesthat is conditioned by the beginning of coagulation process in cells of intermittent decay and at 100 h ageing ofspheroidezation of lamels of αphase. At the same timeplasticity grows that is dislocation occurring at deformation overcome easier obstacles in the form of large coagulated particles of αphase. Thus, decrease of strengthproperties and increase of plasticity at alloy overageing iscaused by growth of distances between earlier formed extractions due to their coagulation, decrease of particlenumber in matrix volume unit and decrease of stressrequired for bypass of particles by dislocations.Intensity of coagulation processes increases at further growth of ageing temperature, for example, at 900or 1000 °С; also the fall of strength properties is observed. Therefore, it is not appropriate to carry out thermalpostprocessing – ageing in specified temperature range.Conclusion Increasing heating time for hardening the deformingstresses fall and in this case plasticity grows that is connected with dissolving of excess αphase. The characterof changing plasticity of samples hardened from 1200 °Сchanges sharply, instead of expected growth of plasticityat heating time increase its fall is observed. It is supposed that it is caused by the presence of «fishscale» fracture in structure.It is not appropriate to carry out hardening of alloy47ХНМ lower than 1225 °С as it results in microcracksformation at deformation decreasing plasticity and viscosity of alloy.Temperature increase more than 1300 °С results insharp decrease of plasticity that is conditioned by flashing of particles of αphase on the basis of Cr and liquidphase spread on grain boundaries.At ageing at 600 °С of hardened alloy 47ХНМ at ageing time increase the strength properties grow insignificantly that is conditioned by decay inside αphase.At growth of alloy ageing temperature to 700 °С atinitial stages of ageing the increase of resistance to smallplastic deformations is noted. Its value increases sharplyat further rise of ageing time. Strengthening is caused bydiscontinuous precipitation of αphase in γmatrix.At ageing temperature 800 °С strength propertiesdecrease at increase of ageing time that is conditionedby the beginning of coagulation process in cells of intermittent decay and at 100 h of ageing by spheroidezationof lamels of αphase.Thus, in order to obtain high strength propertieswith minimal level of elastic defects the thermal treatment of alloy should be carried out in temperature range 650...750 °С and ageing time should amount from 8to 20 h depending on ageing temperature.Bulletin of the Томsк Pоlytеchnic University. 2007. V. 311. № 2116REFERENCES1. Rakhshtadt A.G. Spring steels and alloys. – Moscow: Metallurgiya,1971. – 496 p.2. Solonin M.I., Kondratiev V.P., Votinov S.N. Alloy ХНМ1 as a perspective material for elements of construction of nuclear and thermonuclear reactors with water coolant // VANT. Series Science ofmaterials and new materials. – 1995. – Issue 1(52). – P. 13–20.3. Skakov M.K., Mukazhanov E.B., Akhmetzhanov B.K. Phasestructural changes in precipitation hardening alloy 47ХНМ after hardening // Bulletin of National Academy of science of RK. Chemicalseries. Almaty. – 2006. – № 2(356). – P. 75–78.Received on14.11.2006

Features of change of mechanical properties of precipitation hardening alloy 47ХНМ at hardening and ageing

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