Materials Science and Engineering A323 (2002) 285–292

Precipitation in high strength low alloy (HSLA) steel:a TEM study

S.K. Mishra(Pathak) *, S. Das, S. RanganathanNational Metallurgical Laboratory, Jamshedpur 831007, India

Received 22 March 2000; received in revised form 28 March 2001

Abstract

The HSLA-100 steels contain various alloying additions such as Cr, Mn, Mo, Cu and Ni apart from niobium and carbon.Precipitation of carbonitrides in these steels is complex in nature due to several elements with affinity for carbon and nitrogen.This phenomenon plays a significant role in the microstructure evolution of these steels during thermomechanical processing. Theprecipitates formed in a HSLA-100 steel containing Cr0.58, Mn0.87, Mo0.57, Nb0.032, Ni3.54, Cu1.98, C0.02 at different temperatureswere studied using transmission electron microscope. The selected area diffraction and EDS analysis were used to identify theprecipitates. The investigation showed that several complex precipitates were present in the steel. The type of precipitates and theirmorphology and the relevance of these precipitates to the design of HSLA steels are discussed in this communication. © 2002Elsevier Science B.V. All rights reserved.

Keywords: HSLA steel; Precipitation; TEM

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1. Introduction

Precipitation of carbonitrides is an important phe-nomenon influencing the microstructure of highstrength low alloy (HSLA) steels and, therefore, themechanical properties. Considerable efforts are ex-pended the world over to understand the precipitationphenomenon in these steels. This is crucial for a suc-cessful design of the alloys and of the thermomechan-ical treatment to be adopted in order to achieve thedesired mechanical strength. Several studies have beenreported in literature on the prediction of the chem-istry and quantity of the precipitates formed in thesesteels at different temperature [1–8]. Attempts havealso been made to characterise these precipitates usingoptical microscopy and other techniques [9–13]. Ti,Nb, and V are most commonly used as the alloying

elements to precipitate as carbonitrides and inducegrain refinement in these steels. Apart from theseother elements such as Mo, Cu, Ni are added tothese steels making the chemistry complex. In addi-tion to these, Al is present in these steels resultingfrom the steel making process. The presence of thesealloying elements makes the precipitation behaviourvery complex. A comprehensive understanding of theprecipitation behaviour is essential to achieve the de-sired properties.

HSLA-80 and 100 steels are being developed appli-cations in naval structures. These steels contain vari-ous alloying elements such as Cr, Mn, Mo, Cu andNi apart from Nb and C and other trace elements.The copper is added for precipitation strengtheningwhereas few have strong affinity for carbon and ni-trogen and to form their carbonitrides. Hence, theprecipitation in these steels is complex in nature. Mi-crostructural evolution during austenite decompositionin HSLA-80 steels at different temperature has beenreported in literature [14,15]. The complex precipita-tion in HSLA-100 has not been explored much.

* Corresponding author.E-mail address: suman@csnml.ren.nic.in (S.K. Mishra(Pathak)).

0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0 9 21 -5093 (01 )01382 -X

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Table 1Composition of steel

P S Si Cu Ni Cr Mo AlC NbMn

0.003 0.006 0.25 1.98 3.54 0.58 0.570.02 0.0380.87 0.032

Fig. 1. TEM of extraction replica of as-received sample, showingrectangular rod type precipitates containing Al, Nb, Mo.

formation of carbonitrides. Therefore, the cationicsublattice of the carbonitride precipitates will containmore than one alloying element. However, the studiespredicting the precipitation phenomena in these steelshave usually adopted a simple picture of the cationicsublattice containing only one or two of the majoralloying elements, usually chosen from Ti, V and Nb.It will be very useful to actually characterise theseprecipitates and analyse the chemistry of these and toexplore the possibility of the other alloying elementsoccurring in the cationic sublattice. Formation of in-termetallics, the precipitation of aluminium nitrideand the presence of Fe in the carbonitride phase areother related phenomena, which can influence the pre-cipitation characteristics of these steels. Hence ananalysis of these also is important in this context.Studies reported in the literature [9–13] have usuallydescribed the precipitation behaviour in samples agedat low temperatures prepared from hot rolled steel.These studies show that the precipitates are complexin chemistry containing several alloying elements inthe cationic sublattice. These studies used samplestreated at the respective temperature for a short inter-val of time. The present investigation was carried outto study the chemistry of the precipitates at equi-librium with the matrix at different temperature from1000–1200 °C.

1.1. Complex precipitation

The HSLA steels contain several alloying elements,which can form various types of precipitates. The ele-ments, Mo, Ti, V and Nb have an affinity for the

Fig. 2. TEM of extraction replica of as-received sample showing (a) AlN precipitates and (b) corresponding SAED pattern.

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Fig. 3. TEM of thin foil of as-received sample showing (a) lath typestructure and (b) precipitates.

Fig. 4. TEM of extraction replica of 1000 °C treated sample showingdifferent precipitates.

Fig. 5. TEM of extraction replica of 1000 °C treated sample showing(a) MoTiC2 precipitates and (b) corresponding SAED pattern.

2. Experimental details

The steel under study was obtained from the officeof Naval Research, USA. The steel used in thepresent investigation was melted at 1700 °C. It wasvacuum degassed, followed by argon stinting and cal-cium silicide injection. The steel was bottom pouredfrom the degas ladle and cast into ingots. The slabswere reheated and soaked for 1 h at 1150 °C andtaken to a roughening mill at 1230–1260 °C. The 2×96×340 in. size pieces were austenised at 900 °C for2 h and water quenched. The plates were shipped aswater quenched and were used in this study. Thecomposition of the steel is given in Table 1. Speci-mens were cut from bulk into 15×10×8 mm sizesfor heat treatment. The samples were equilibrated for1 h each at different temperatures ranging from 1000

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Fig. 6. TEM of thin foil of 1100 °C treated sample showing lath-boundary precipitates.

were carried out at 200 kV using a Philips CM200TEM with EDAX DX-4 EDS spectrometer for ele-mental analysis. The microstructural characterisationwas carried out in bright field mode. The selectedarea diffraction and in-situ EDS analysis were carriedout to identify the precipitates. EDS analysis obtainedwere the representative of the elements present andwere qualitative.

3. Results and discussion

The received samples showed three major types ofprecipitates. EDS indicated that the rod shaped pre-cipitates (Fig. 1) in these samples contained Al, Nb,Mo and Cr in the ratio of 55:17:13:3.5, approxi-mately. In these studies, the analysis of the anionicsublattice could not be established directly since theEDS is not sensitive to the analysis of C and N,which are the primary constituents of the anionicsublattice in these precipitates. SAED analysis ofthese precipitates could not confirm the identity ofthe particle. It is possible that these are mixture of(Al, Nb) and (Al, Mo) carbides or the complex car-bides containing Al, Nb, Mo. Presence of AlN pre-cipitates of sizes in the range of 0.2–0.3 �m werefound and could be confirmed by SAED and EDSanalysis.The SAED pattern analysis show the hexago-nal phase of AlN with lattice parameter as a=3.110A, b/a=1.0, c/a=1.6 and �=120. EDS show pres-ence of Al only. The received samples show the pres-ence of AlN precipitates in circular and rectangularmorphologies. The microstructure and SAED patternof one such precipitate are given in Fig. 2. In addi-tion to these, particles rich in Nb were also detected.

to 1200 °C under argon (IOLAR-1) atmosphere andwere quenched in water. The samples were preparedby the carbon extraction replica method on metallo-graphic samples for analysing the precipitates. Repli-cas were taken out carefully on copper grid by deepetching. A few thin foil specimens were also made.Thin foil specimens for Transmission Electron Micro-scope (TEM) were prepared by punching 3 mm discsfrom the wafers cut from heat treated samples.Wafers were mechanically ground by hand to around100 �m before punching. These discs were twin jetelectropolished to electron transparency in a mixtureof perchloric and glacial acetic acid. TEM studies

Fig. 7. TEM of extraction replica of 1100 °C treated sample showing (a) Al5Fe2 precipitates and (b) corresponding SAED pattern.

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Fig. 8. TEM of thin foil specimen of 1100 °C treated sample showing(a) lath structure and (b) fine precipitate and dislocations.

Fig. 9. TEM of extraction replica of 1150 °C treated sample showing(a) Al–Fe precipitates and (b) corresponding SAED pattern.

tates were found to be in the size range of 0.1–0.3�m. It was observed that the sample quenched from1000 °C has different precipitates. (MoTi)C2 precipi-tate, cubic a=4.313 A, was confirmed throughSAED pattern analysis. The EDS show the com-position as Al0.4Nb0.4Mo47.7Ti42.9Cr1.5Mn1.3Fe0.7, allweights are given in atom percent. In all the investi-gations EDS results were taken a qualitative one. Moand Ti were nearly in the ratio 1:1 in EDS analysis.This is consistent with the analysis of the SAED pat-tern which showed these precipitates to be (MoTi)C2.(MoTi)C2 precipitates were in the range of 0.5–0.6�m in size and were rectangular and distorted circularin shape. Fig. 5 shows the typical microstructure andindexed SAED pattern of a (MoTi)C2 precipitate.Many precipitates of AlN in the range of 0.2–0.25�m were also seen in the extraction replica. The EDSanalysis of these precipitates shows the presence of Alonly.

These contained Fe and Ti also. Though not reportedin Table 1, trace amounts of Ti were present in thissteel. Some precipitates containing Nb as major con-stituent along with Mo were also observed. TheSAED of the Nb rich precipitates matched well withthat of Niobium carbide (NbC). Mo may be formingcomplex carbide and is in solution of NbC andthereby forming a (Nb, Mo)C. The close similaritybetween the patterns of carbide and carbonitride pre-clude any conclusive assessment of the chemistry ofthe precipitate. The microstructure of the thin foilspecimen of the received samples is shown in Fig. 3.A lath-type structure is seen and also rectangular andcircular type precipitates are clearly visible.

Fig. 4 show the microstructure of the precipitatesfor the sample quenched from 1000 °C. The precipi-

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Fig. 10. TEM of extraction replica of 1200 °C treated sample showingfine precipitates (a, b).

Fig. 11. TEM of extraction replica of 1200 °C treated sample showing(a) Mo3N2 precipitates and (b) corresponding SAED pattern.

The extraction replica for the samples quenchedfrom 1100 °C showed the presence of AlN. Fine pre-cipitates of the order of 100 nm near the lathboundary were observed Fig. 6. They were very fineand hence no SAED pattern of these could be ob-tained. The EDS analysis of the cluster of precipitatesshowed the presence of Mo, Ti, Nb, Al, Cr, Fe. It ispossible that the precipitate containing Mo and Tiwhich were observed at 1000 °C are gradually goinginto solution and hence they are becoming finer too.Beside these, precipitates of intermetallic phase Al5Fe2,tetragonal a=7.675 A, b/a=0.834, c/a=0.547, werefound. One such precipitate along with the SAEDpattern is shown in Fig. 7. These were about 0.3 �min size and have elliptical and rectangular morphology.

The EDS analysis showed the composition as Al88.6

Nb0.6Mo3.2Ti0.5Cr1.5Mn0.5Fe4.9Ni0.3. Lath martensitetype microstructure was seen in thin foil specimens.Very fine precipitates and high density of dislocationswere also seen and are shown in the Fig. 8.

Al–Fe precipitates, cubic a=5.8 A, were found in thesamples prepared from 1150 °C. Fig. 9 shows the mi-crostructure and SAED pattern of the precipitate. EDSanalysis shows the qualitative composition asAl50.6Nb0.5Mo1.8Cr0.6Fe41.4Ni2.0. Similar compositionswere also obtained for different precipitates whoseSAED patterns were matching with AlFe phase. SAEDanalysis and EDS analysis in terms of Al and Fe ratiowere very near. Hence it was concluded that these wereAlFe phase precipitates. These precipitates were found to

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Fig. 12. TEM of thin foil of 1200 °C treated sample showing (a) lathstructure and (b) very fine precipitates and dislocation.

Al6.6Nb5.1Mo2.9Ti4.5Cr48.4Mn10.6Fe23.8Ni6.1. Precipitatesof Mo3N2 phase, cubic a=4.650 A were found in thesesamples. The SAED pattern matched well with the cubicMo3N2 phase but it also nearly matched with hexagonalMo7Nb3C4. The EDS analysis showed the presence ofMo79Nb5.1Al3.6Ti0.5Cr7.4Mn0.9Fe2.9Ni0.7. On the basis ofSAED better matching it was deduced that these precip-itates are cubic Molybdenum nitride with niobium assolid solution in the matrix. However, the presenceMoNb complex carbonitrides or carbides can not beruled out. Fig. 11 shows the microstructure and corre-sponding SAED pattern. Thin foil specimen showed thefiner precipitates and presence of dislocations (Fig. 12).It is clear that precipitates are fewer for the sampleprepared from 1200 °C compared with 1100 °C or asreceived samples.

The TEM studies reported here show that the precip-itates formed are complex in nature containing severalalloying elements in the cationic sublattice of the precip-itates. Since Mo and Cr have an affinity to form carbidesand nitrides, their presence in the precipitates studied isnot surprising. It is also apparent that different types ofthe precipitates with different chemistry and crystallo-graphic structure are formed in these steels. In additionto these, intermetallic compounds and simple precipitatessuch as Fe7C3 have been detected. Therefore, a simplepicture of precipitation in these steels focussing only onthe niobium or titanium carbonitrides is not adequate torepresent the precipitation behaviour in this class of steel.Any realistic modelling and prediction of the precipita-tion behaviour during thermomechanical treatment hasto take into consideration the complex precipitationscenario as reported here. The Nb rich precipitatesdetected in the as received sample have not been detectedin the samples quenched from higher temperatures. Thisis consistent with the predictions made on the solubilityof this precipitate in the steel. AlN has been detected inall the samples except for the samples quenched from1200 °C AlN precipitates are expected to dissolve in thematrix at about 900 °C. Their presence in these steelseven at higher temperature calls for further investigation.These samples were soaked at prescribed temperatureonly for 1 h. A longer soaking time can possibly ensurethat these particles were dissolved completely. It is alsopossible that these particles were precipitated duringquenching from high temperatures.

4. Conclusion

Precipitates formed in a HSLA steel quenched fromvarious temperatures were studied using TEM. Theprecipitates formed are complex in nature containingseveral alloying elements in the cationic sublattice. Inaddition to carbonitrides, intermetallic compounds werealso detected in the steel. The studies show that a simple

be of the order of 0.5–1.0 �m in size. A few precipitateswith Fe as major constituent were also found whoseSAED pattern matches with Fe7C3. These particlescontained Fe and Cr in the ration of 67:9. It may be notedthat formation of similar precipitates with varying ratioin the cationic sublattice in the Fe�Cr�C system has beenalready reported. Some AlN precipitates of 0.1–0.2 �msizes were also observed.

Replicas prepared from the samples quenched from1200 °C showed fewer precipitation. AlN precipitateswere not observed in this case. Very fine precipitates ofsize ranging between 10–100 nm were seen (Fig. 10).Again SAED pattern could not be obtained from theseparticles. The EDS analysis showed the presence of

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picture of the precipitation behaviour is not adequate todescribe the precipitation phenomena in these steels. Thecomplex precipitation behaviour has to be taken intoconsideration while designing the alloy and the thermo-mechanical treatment.

Acknowledgements

The authors are grateful to the office of the NavalResearch, USA for financial assistance to carry out thiswork under grant number N00014-95-1-0015.

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