Effect of addition of nitrogen on the microstructure and properties of cold-forg

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

Abstract: The heat-treatment characteristics,mechanical properties,and wear behavior of four cold-forged-roll steels were studied and their microstructural characteristics were analyzed using transmission electron microscopy(TEM),scanning electron microscopy(SEM),X-ray diffractometer (XRD),and an oxygen-nitrogen analyzer.The results show that the introduction of nitrogen facilitates improvement in the mechanical properties of cold-forged rolls.A solid-solution strengthening effect is obtained by interstitial nitrogen,and the hardenability of rolls can be improved as well.A significant effect on grain-size refinement is obtained by highly thermally stable fine M(C,N) carbonitrides,which pin the grain boundaries during austenitization and block the growth of austenitic grains.Compared to conventional roll steels,the optimum quenching temperature of steel with added nitrogen can be increased by 20-30 ℃.Therefore,the hardening temperature range is expanded,which is beneficial to the heat-treatment process.Furthermore,high-hardness carbonitrides can improve the wear performance.

Key words: carbonitride; cold-forged roll; microstructure; mechanical property; wear behavior

1 Introduction

Using nitrogen as an alloying element of iron-based alloys has been profoundly studied since the beginning of the last century[1].Typically,nitrogen is used in austenitic stainless steel to replace nickel and to enhance the stability of austenite along with the strength and corrosion resistance of steel[2-3].Furthermore,in a high-strength low-alloy (HSLA) steel,nitrogen is added as a micro alloying element that binds with other alloying elements such as van-adium,niobium,and titanium,to form various carbon-itrides[4].Enhancing nitrogen increases the super-saturation of ferrite of HSLA steels and promotes a more active nucleation of carbonitride particles[5].Due to the smaller size and better thermal stability of carbonitrides,the strength properties of HSLA steels can be improved at low cost.

Recently,some studies have been conducted to add nitrogen into tool and die steels[1,6-8].Not only can nitrogen produce marked solid-solution hardening and precipitation-strengthening reactions in these steels,but it can also improve their wear resistance,which is a crucial property for tool and die steels.

With the ongoing and rapid improvements in rolling technology and the higher quality require-ments of rolled products,roll technology is becoming increasingly important.Since the last century,research-ers have extensively studied ways to obtain high-performance rolls[9].The effect of the addition of nitrogen on the heat-treatment characteristics,mech-anical properties,and wear resistance of cold-forged-roll steels were investigated in this paper.

2 Experimental procedures

2.1 Materials preparation

Four types of rolled steels were selected for this study:the Cr5 and SH types,each with and without the addition of nitrogen.These steels were fabricated in a 500 kg medium-frequency furnace according to the designed chemical composition shown in Table 1.Steel ingots were forged into several round bars with subsequent spheroidized annealing,quenching,and tempering processes.

Table 1 Chemical composition of cold-forged-roll steels %

2.2 Heat-treatment characteristics

Continuous cooling transformation (CCT) diagrams were generated using a THERMECMASTOR-Z ther-mal simulator with specimens 8 mm in diameter and 12 mm in length.A saturated aqueous picric acid solution plus a small amount of wetting agent was used to observe the prior-austenite-grain boundaries of the steels after quenching at various austeniti-zation temperatures from 950 to 1 050 ℃.The average grain size was estimated based on the ASTM E112-96(2004) standard.

2.3 Mechanical properties testing

The tensile strength and impact toughness of the four steels in various quenching and tempering con-ditions were evaluated according to GB/T 228-2002 and GB/T 229-2007,respectively.The hard-ness of the specimens after various heat-treatment processes was tested on a Rockwell hardness tester.

2.4 Wear testing

A Falex block-on-ring friction and wear testing machine was used to evaluate the wear resistance of the four steels,which had a similar HRC value (about 60).The block was the test specimen and the ring was a friction pair consisting of GCr15 steel.The block and ring were put under pure sliding contact by oil lubrication,with a contact load of 1 000 N and a sliding speed of 0.366 m/s.Each test took two hours to ensure that the sample was in a stable wear condition.The wear resistance of the steels was evaluated using the specific wear rateK,which was calculated using the following equation:

(1)

where,Vis the volume loss;Lis the load;andDis the sliding distance.

2.5 Microstructure characterization

To determine the percentage of each component present in the steel,the total and combined nitrogen contents were analyzed using the oxygen-nitrogen analyzer.All the precipitates in the steel were obtained by electrochemical extraction and then evaluated in an X-ray diffractometer (XRD).The morphology and com-position of the carbides and carbonitrides were analyzed by transmission electron microscopy (TEM) with selected area electron diffraction (SAED)and scanning transmission electron microscopy (STEM) with energy-dispersive spectrometer (EDS).

3 Results

3.1 CCT diagrams

Fig.1 shows CCT diagrams of the four steels.Due to the large volume of alloy elements such as Mo and V,the hardening ability of the SH and SHN steels was greatly improved relative to those of the Cr5 and Cr5N steels.Furthermore,when comparing the Cr5N and Cr5 steels,the critical cooling rate of pearlite decreased from 2 K/s (for Cr5N) to 1 K/s (for Cr5).A similar result was obtained when com-paring the SHN and SH steels,which indicates that the hardenability can be improved by the addition of nitrogen.Fig.2 shows the microstructures of the four steels as they were cooled to room temperature at 0.3 K/s.At this cooling rate,the Cr5 steel obtained a very coarse lamellar pearlite,the Cr5N steel obtained fine pearlite,only some pearlite formed in the SH steel,and pearlite just began to form in the SHN steel.It can be concluded that the addition of nitrogen postpones the transformation from austenite to pearlite.Table 2 shows the critical transformation points of the four steels,whereby the eutectoid transformation temperature was increased by the addition of alloy elements such as Mo and V,but decreased by the addition of nitrogen.Therefore,both the Cr5N and SHN steel have a lower martensitic transformation temperature(Ms).

3.2 Grain-coarsening temperature

Table 3 shows the austenite-grain size of the four steels at various austenitization temperatures,in which it can be seen that the austenite grains coarsen gradually with increases in the austeni-tization temperature.Fine grains could be obtained in all the four steels when the austenitization tem-perature was below 1 000 ℃.Once the austeni-tization temperature rose above 1 000 ℃,however,difference in the grain sizes could be easily distinguished and relatively finer grains could be observed in the steel to which nitrogen had been added (Cr5N,SHN).Fig.3 shows the original austenite-grain microstructures of the four steels after quenching at 1 050 ℃.The SHN steel,which contains nitrogen,could maintain an average grain size of 9.5 or more,whereas the grain size of the common SH steel reached only 8.5 with some coarse grains larger than 25 μm in diameter.

Table 2 Critical transformation points of the four steels under study ℃

3.3 Quenching hardness and tempering stability

Fig.4 shows the hardness of the four steels at dif-ferent quenching temperatures ranging from 950 to 1 050 ℃.It can be seen that as the temperature increased,the hardness initially increased to a peak value and then decreased slowly.Comparing the Cr5 and Cr5N steels,we find that the hardness of the Cr5N steel was higher than that of the Cr5 steel when the hardening temperature was below 960 ℃or over 1 010 ℃.Similar behavior is observed when comparing the SH and SHN steels.

Fig.5 shows the tempering hardness of the four steels after quenching at 1 050 ℃.The tempering temperatures ranged from room temperature to 700 ℃.Very different behaviors were observed for the Cr5 and SH-type steels with respect to their tempering stability.For the Cr5 type steel,a greater tempering hardness could be obtained by the addition of nitrogen when the tempering temperature exceeded 500 ℃.Due to their higher alloy contents,both the SH-type steels with and without the addition of nitrogen showed significant secondary-hardening behavior when tempered at 480-520 ℃,and even greater hardness could be observed in the SHN steel when tempered at a higher temperature.As such,it can be ascertained that there is no significant improve-ment in hardness during the secondary-hardening process by the addition of nitrogen,but it can slow down the rate of decrease in hardness in subsequent higher-temperature tempering.

Table 3 Average austenite-grain size of the four steels under study

Fig.4Hardnessofthefoursteelsunderstudywhenquen-chedatdifferenttemperatures

Fig.5Hardnessofthefoursteelsunderstudywhentem-peredatdifferenttemperatures(quenchedat1050℃)

3.4 Mechanical properties

Fig.6 shows the mechanical properties of the four steels after they had undergone the same quenching and tempering processes.The results indicate that,on average,the microalloying SH-type steel with the added nitrogen experienced an increase in yield strength and ultimate tensile strength of about 6% and 4%,respectively,with no obvious changes in toughness or ductility.Thus,it can be concluded that the addition of nitrogen can significantly improve the strength,especially the yield strength,of the steel with only relatively small losses in toughness and ductility.

Fig.6Mechanicalpropertiesofthefoursteelsafterthequenchingandtemperingprocesses

3.5 Wear behavior

Table 4 presents the specific wear rates of the four steels,in which it can be seen that those with nitrogen added exhibited lower wear rates.Fig.7 shows the friction coefficients obtained for the sliding wear process,in which it is clear that those to which nitrogen had been added exhibited a lower friction coefficient.

Table 4 Wear resistance of the four steels under study

Fig.7Frictioncoefficientsduringthewearslidingprocess

4 Discussion

4.1 Nitrogen in cold-forged-roll steels

In cold-forged-roll steels,nitrogen exists in two forms:free nitrogen (solid solution) and combined nitrogen (nitrides and/or carbonitrides).To deter-mine the percentage of each,the total and combined nitrogen were analyzed using an oxygen-nitrogen analyzer,the results of which are shown in Table 5.It can be seen that with increases in the quenching tem-perature,the content of the combined nitrogen de-creased and dissolved into the steel matrix.Fig.8 shows the XRD structural analysis results of the carbides and carbonitrides in the four steels for which the quenching temperature was 1 000 ℃ with sub-sequent tempering at 150 ℃.There were MC carbides and M(C,N) carbonitrides present in SH and SHN steels,respectively,whereas only M7C3carbides were found in the Cr5 and Cr5N steels due to the low content of V alloy.

4.2 Mechanism of nitrogen-alloyed strengthen-ing in cold-forged-roll steels

Interstitial elements (N,C,and B)can increase the strength of steels,and of these,nitrogen is the most effective one[10].The powerful strengthening effect of interstitial nitrogen is illustrated in Fig.6,whereby the yield strength and ultimate tensile strength of the steel are improved to a large degree.

Table 5 Nitrogen content distributions in the steels under study

Fig.8XRDstructuralanalysisofthecarbidesandcarboni-tridesinthesteels

Nitrogen is also a strong austenite stabilizer element that reduces the eutectoid transformation and theMstemperatures of steel,as shown in Table 2.Greater hardness can be obtained at relatively low-temperature quenching because of the decreased eutectoid transformation temperature[11].A greater hardness can also be obtained at relatively high-tem-perature quenching because of the grain refinement effect.Therefore,the hardening temperature range is expanded by the addition of nitrogen,which is beneficial to the heat-treatment process in plant production.

The significant effect on grain-size hardening by the addition of nitrogen can be described using the well-known Hall-Petch equation.The main reason for the grain refinement effect is the fine distri-bution of carbonitrides.Fig.9 shows the morpho-logies of carbides and carbonitrides in Cr5 and SHN steels.Only M7C3spherical carbide particles are embedded in the martensite matrix of Cr5 steel,with diameters ranging from 500 nm to about 1m.The presence of MX particles in SHN steel was con-firmed by STEM with EDS,as shown in Fig.10.Compared with M7C3carbides,carbonitrides are typically smaller in size.However,some large carbonitrides can also be observed in SHN steel.M(C,N) carbonitrides have high thermal stability and cannot be dissolved during high-temperature austenitization.Austenite grains can be pinned by these finely distributed particles and restricted in their growth during the austenitization process[12-13].This characteristic of M(C,N) carbonitrides enables the use of a higher quenching temperature for SHN steel without obtaining coarse austenite grains.Compared to that without added nitrogen,the same grain size can be obtained in nitrogen-alloyed steel by increasing the austenitization temperature by 20-30 ℃.For the Cr5N steel,a small number of carbonitrides can be found due to the relative lower content of V alloy,which cannot offset the deficiency of the drastically dissolved carbides in the high-temperature austenitization process.There-fore,the grain refinement effect is weaker than for SHN steel.Furthermore,the high-hardness values of M(C,N) carbonitrides have a positive effect on the wear resistance of cold-forged-roll steel during lubricated sliding.In this case,the matrix of the cold-forged-roll steel can be well protected from sliding wear.

Fig.9Imagesshowingmorphologiesofcarbidesandcarboni-tridesinthesteelsunderstudy:TEM+SAED

5 Conclusions

Nitrogen is present in cold-forged-roll materials in two forms:free nitrogen (solid solution) and combined nitrogen (nitrides and/or carbonitrides).Both forms play an important role in improving the mechanical properties of cold-forged-roll steels.

A powerful solid-solution strengthening effect can be obtained by the presence of interstitial nitrogen,which can also improve the hardenability of the rolls.A significant effect on grain-size refinement can be obtained by highly thermally stable fine M(C,N) carbonitrides,which pin the grain bound-aries during austenitization and block the growth of austenitic grains.Compared to conventional cold-forged-roll steels,the optimum quenching temper-ature of the steel with added nitrogen can be increased by 20-30 ℃.Significant secondary-hardening behavior can be obtained during subsequent 480-520 ℃ tempering and higher-temperature quenching.Therefore,the hardening temperature range is expanded by the addition of nitrogen,which is beneficial to the heat-treatment process in plant production.Furthermore,a positive effect on the wear performance of cold-forged-roll steel can be obtained by high-hardness M(C,N) carbonitrides.