Effect of induction heating and tempering on properties and microstructures of q

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Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: In this study,a C-Mn quenched steel tube was quickly tempered by induction heating,and the influence of the tempering temperature on its performance was studied and compared with that by traditional tempering.The results show that the yield strength of both is quite strong with regular changes in the tempering temperature,but that the tensile strength of the tube tempered by induction heating is higher than that tempered by traditional tempering by about 25 MPa,and the elongation after induction tempering is significantly higher than that after traditional tempering.The differences in the microstructures of tubes after induction and traditional tempering were compared by metallographic microscope,scanning electron microscopy,and transmission electron microscopy.Theoretical analysis was also performed.Compared with traditional tempering,a fine dispersion of precipitated carbides occurs after induction tempering,which is the main reason for the performance differences.

Key words: induction heating; tempering; quenched steel tube; property and microstructure

1 Introduction

With the ongoing development of China’s iron and steel industry,the requirements of industrial produc-tion regarding the quality of mechanical products,energy saving,reduced energy and resource con-sumption,and environmental protection are ever increasing.The quality of metal materials is closely related to the heating technology employed.Induction heating technology has the advantages of high heating efficiency,environmental protection,flexible switching,and reduced equipment use.This technology is widely used in metal material process-ing,production,and other aspects,and its market application prospects will continue to broaden[1-6].

Induction heating[7-9]is also widely used in the manufacture of seamless steel pipes,primarily for quenching and rarely as a tempering heat treatment.Tempering as a heat treatment is a key process for controlling the microstructures and properties of the materials,and requires a highly uniform heating temperature.For steel pipe products,the temperature fluctuation over their entire length is usually con-trolled to within ±5 ℃,with some high-end prod-ucts even requiring a narrower range of ±3 ℃.The final mechanical properties of the hardened steel depend on the tempering process used.The normal tempering procedure requires a strictly controlled tempering temperature as well as long tempering and holding time,generally from dozens of minutes to several hours.This long heat-preservation period consumes much heat energy and lowers production efficiency.

The use of rapid induction tempering can greatly reduce the required heating and holding time,and represents an effective way to save energy in heat treatment.This technology has great economic value,as well as theoretical significance,as the induction-heat tempering of large-scale seamless steel pipes is a new field.This paper presents an analysis of the dif-ferences in the properties and microstructures obtained by induction and traditional tempering.

2 Test materials and methods

For the test,a 244.5 mm×14.84 mm quenched tube was used,with the length of 1 200 mm,and the material chemical composition of 0.25C-1.2Mn-0.4Cr.Table 1 shows the parameters used in the induction and traditional tempering processes.The induction-heat temperatures applied were 550,600,650,and 700 ℃,and those applied in the traditional furnace heating were 550,580,610,640,and 670 ℃.

Table 1 Induction and traditional tempering process parameters

To accurately measure the circumferential and axial temperature changes of the tube during induction tem-pering,buried thermocouple test measurements were conducted at different locations on the inner tube wall (550,600,and 650 mm from the left side of the tube),as shown in Fig.1.Fig.2 shows the temperature curves obtained during induction heating (heated to 600 ℃),with the axial and circumferential temperature dif-ference controlled to within ±10 K.

Fig.3 shows the hardness distributions at the different temperatures from the outer to inner walls.The hardness distributions of the inner and outer walls were the same,which indicates that the heating uniformity for this wall thickness was good.

After mechanical grinding,the sample was polished to achieve a mirror surface and was then corroded with 4% nitric acid alcohol solution.The metallographic microstructure of the sample was then observed using an optical microscope,and the microstructure was examined and analyzed using a metallographic fiber microscope,EVO MA25 scanning electron micro-scope,and JEM 2100F transmission electron micro-scope.A tensile test was performed on an MTS 810-15 testing machine,and the impact performance was determined using the JBN-300B equipment according to the ASTM standard 370-2010.The Charpy V-notch impact energy of the material was also determined for a sample size of 10 mm×10 mm×55 mm.

3 Comparative study of properties obtained via induction and traditional tempering

Figs.4-7 show comparisons of the results obtained by induction and traditional tempering.As the temperatures used in both the induction and traditional tempering processes increased,the yield and tensile strength decreased according to the same change rule (Fig.4).As shown in the figure,for every 10 K increase,the yield strength decreases by about 20 MPa,and the tensile strength decreases by about 16 MPa.The yield strength values are the same,but the tensile strength after induction tempering is about 25 MPa higher than that after traditional tempering,so the yield strength ratio obtained by induction tempering is lower than that obtained by traditional tempering,which improves its safety of use.The elongation obtained by induction tempering is signif-icantly better than that obtained by traditional tempering,with an absolute value increase of 5%,and a relative increase of more than 40% (Fig.5).After 650 ℃ induction heating,the strength obtained was close to the center line of 80-grade steel,and after 600 and 700 ℃ induction heating,the strength obtained was at the upper and lower limits of 80-grade steel,respectively,which means the heating temperature can meet the requirements of 80-grade steel within a temperature range of 100 K.After induction heating at 550 ℃,the strength was close to the center line of 110-grade steel.

The influences of the induction and traditional tempering temperatures on the impact toughness were found to be consistent.With increases in temperature,the impact toughness was observed to increase.Specifically,the impact toughness increased by about 8 J for every 10 K increase in tem-perature,and the relationship between the two was consistent (Fig.6).Fig.7 shows the ductile-brittle transition curves of the two tempering processes,in which it can be seen that the ductile-brittle tran-sition temperatures ranged between30 and40 ℃,with no significant difference in their impact tough-ness values at each test temperature.This finding is inconsistent with the induction tempering results reported in literature[10],in which the authors found that induction tempering could significantly reduce the ductile-brittle transition temperature.

Fig.7Ductile-brittletransitioncurvesafterinductionandtraditionaltempering

4 Comparative study and analysis of micro-structures after induction and traditional tempering

In the quenching state,the primary microstructure of steel is martensite or martensite+retained aus-tenite (Fig.8),both of which are in a metastable state,with the martensite in a carbon supersaturated state and the retained austenite in a supercooled state.Both these microstructures tend to transform into a stable state of ferrite plus cementite active carbide.It is difficult to achieve atomic diffusion at room tempera-ture,so tempering is used to promote microstructural transformation and to eliminate internal residual stress and improve the strength-toughness relationship.

Fig.9 shows images of the microstructures ob-served by metallographic microscope and scanning electron microscopy after induction and traditional tempering (550 ℃),respectively.With respect to the metallographic microstructure,the microstruc-ture of the induction-tempered strip is more obvious than that of the traditionally tempered strip.When the tempering temperature was increased from 550 to 700 ℃,the ferrite gradually recrystallized,losing its original sheet or strip shape to exhibit polygonal grains.The induction-tempering time is signif-icantly shorter than that of traditional tempering,so the ferrite does not recover and recrystallize fully.However,the microstructure of the traditionally tempered strip is not obvious.

Carbide precipitation is an important microstruc-tural transformation that occurs during the temper-ing process.Its precipitation,transformation,com-bination,and growth run throughout the entire process of heating and heat preservation.In Fig.9,it can be seen that compared with traditional tem-pering,the carbide precipitate in induction tem-pering is finer and better dispersed,and there is a lesser amount of carbide.In Fig.10,it can be seen that the microstructure after the traditional temper-ing process is tempered lath martensite+precip-itated cementite.The distribution of cementite in the microstructure is more uniform,mainly taking the shape of small bars and a few auxiliary par-ticles.The cementite precipitated from the marten-sitic lath boundary is in the shape of a large long strip.

The morphology of cementite in the induction-tempered microstructure is mainly round particles,supplemented by strip (bar) shapes.The cementite particles precipitated at the lath boundary are mainly discontinuous and fine,and the dislocation density of the microstructure is greater than that obtained by the traditional tempering process.

In traditional tempering,the supersaturated carbon in the martensite precipitates in the form of cemen-tite.During the precipitation process,the precipitate grows along a certain crystal surface and crystal direction.In Fig.10,it can be observed that most of the precipitated cementites are bar like and the composition ratio of Fe to C is close to 3∶1.It can also be seen that the carbide precipitation in the traditional tempering process is almost at equilib-rium.In addition,due to the slow heating speed and long holding time of the traditional tempering process,a large number of the dislocations gen-erated in the quenched state will recover,so the dislocation density is low.

In induction heating,because of the fast heating rate and short holding time,dislocations cannot recover.In the process of high-temperature tem-pering,dislocations and grain boundaries are the fast-channel elements in steel.Therefore,carbide precipitation in the induction tempering process is a typical non-equilibrium precipitation.As the holding time is very short,the carbides cannot grow well along a certain crystal surface or crystal direction,so the precipitates are most likely to grow in all directions and become round particles.In addition,due to the short growth time of the precipitates,the growth of the particles is inhibited,so no large strip cementite precipitation occurs at the lath boundary.Although both microstructures comprise tempered lath martensite+tempered cementite,the size,morphology,and distribution of their precipitates obviously differ,which is the main reason for the differences in the properties obtained by induction and traditional tempering.

5 Conclusions

(1) The yield strength obtained by induction tempering is the same as that by traditional tem-pering,but the tensile strength of the former is about 25 MPa higher than that of the latter.The change rules for strength with tempering temperature are the same for both,and the toughness values of the two are the same at room temperature and low temperature.However,the elongation after induction tempering is significantly higher than that after traditional tem-pering.

(2) Induction tempering has the advantages of a fast heating rate,high superheat,a high carbide nucleation rate,and the absence of carbide transfor-mation,consolidation,and growth.Compared with traditional tempering,the main reason for the dif-ferences in the properties obtained by induction and traditional tempering is the reduced total volume fraction and fine dispersion of the precipitated carbide obtained by induction tempering.