Precipitation Behaviors and Strengthening of Carbidesin H13 Steel during Annealing

Ning Angang1,2, Guo Hanjie1,2,+, Chen Xichun3 and Wang Mingbo4

1State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing,30 Xueyuan Rd., Haidian District, Beijing 100083, China2School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing,30 Xueyuan Rd., Haidian District, Beijing 100083, China3Department of High Temperature Materials, Central Iron and Steel Research Institute,No. 76 Xueyuan Nanlu, Haidian District, Beijing 100094, China4Department of Special Metal, Chongqing Materials Research Institute Co., Ltd., 400707, China

Deploying optical microscopy, transmission electron microscopy, electron diffraction and energy dispersive spectrometer analysis. Thisarticle analyze the categories and shapes of carbides of three different positions in H13 ingot after annealing: upside, middle and bottom of ingot.It is found that the microstructure of H13 after annealing is composed of granular pearlite+ small amounts of ferrite and carbide phase. Thecategories of carbides mainly include M23C6 and MC, precipitation temperatures of which are figured out through thermodynamic calculation.Through the test of mechanical properties, it is found sample at the bottom has the optimal mechanical property. Through statistics of amountsand average sizes of precipitates and calculation of precipitation strengthening, it is found that, from upside to bottom of H13 after annealing, thesize of precipitates decreases with increase of precipitation volume fraction, and contributions of precipitates to yield strength enhance gradually.[doi:10.2320/matertrans.M2014452]

(Received December 15, 2014; Accepted January 14, 2015; Published February 27, 2015)

Keywords: precipitates, H13 after annealing, thermodynamic calculation, precipitation strengthening

1. Introduction

H13(4Cr5MoSiV1), adopted by AISI standard, is regardedas a kind of air-cooling hardening hot work die steel whichis widely applied in forging mold, casting mold and hotextrusion mold manufacturing. It has good heat resistance,toughness and hardenability, with the main alloying elementsof Cr, Mo and V.1) Compared with importing H13 steel, thereare still many deficiencies in domestic H13 steel: such asserious banded segregation, cracks appeared when using, lowimpact toughness and so on.2) However, spheroidal annealingprocess can effectively homogenize organization and im-prove the mechanical properties of steel.

Owing to more alloying elements and sophisticatedcomponents in H13, the material organization, especiallycategories of carbides change obviously during heat treat-ment. It is usually considered that there exist MC, M6C,M7C3 and M23C6 carbides during heat treatment. Literature3)

reports that there exist M7C3, M6C and MC without M23C6;Liu Shuxun4) found M23C6 rather than M7C3 in 5% Cr hot-work die steel, while Tsujii5) and Mebarki6) reported thatthere exist M7C3, MC, M23C6 and M6C carbides in 5% hot-work die steel. Some scholars7­9) believe that the reason ofthese difference is related to heat treatment temperatures.M7C3 appears below 873K, and it transforms to M23C6 whentemperature rises above 873K.

Research on effect of alloying elements on H13 steel is notdeep enough. Literature10) regards Cr as the main elementto improve hardness and strength of materials. Cr exists inM7C3 carbides during annealing. However, Dong Jianxin11)

considered that Cr can prevent carbides spheroidizing. Mo

mainly exists in M6C carbides, improving heat resistance andhardenability of steel.12) V can refine grain and improve hightemperature stability of steel, and combine with C to formMC carbides.13)

This article deploys the methods of carbon membraneextraction replica to make TEM (Transmission ElectronMicroscopy) sample in order to observe the morphologiesand categories of precipitates. Through test and statisticsof carbides after annealing, types, particle size distribution,average diameter and the influence of precipitates onmechanical properties are obtained.

2. Experimental Procedure

2.1 Experiment methodsThis experiment deploys the methods of H13 smelting

process, after tapping from EAF (Electric Arc Furnace) andsmelting in LF (Ladle Furnace), VD (Vacuum Degassing)and ESR (Electroslag Remelting), º440mm © 3000mmingot of 14t is available. Forge the ESR ingot to a diameterof 220mm bar stock. The forging temperature is 1373K.Annealing for 15 hours at 1133K, analyze compositions ofnonmetallic elements by using Carbon Sulfur Analyzer andNitrogen Oxygen Analyzer. The compositions of metallicelements are tested by chemical titration. The result is shownin Table 1.

2.2 Sample, sample preparation and testing equipmentsThe sampling position is shown in Fig. 1. Sample from

upside, middle and bottom of bar stock after annealing, thesize of which are º220mm © 60mm respectively. In eachposition, get impact value samples (charpy V), tensilesamples and metallographic samples in 1/2 radius of cross+Corresponding author, E-mail: guohanjie@ustb.edu.cn

Materials Transactions, Vol. 56, No. 4 (2015) pp. 581 to 586©2015 The Japan Institute of Metals and Materials EXPRESS REGULAR ARTICLE

section, the sizes of which are 10mm © 10mm © 55mm,¯8 © 120mm (gripping end is ¯12 © 50mm) and 10mm ©10mm © 10mm. The metallographic samples are numberedas 1#(upside), 2#(middle) and 3#(bottom).

Using carbon extraction replica to make sample prepara-tion, the specific step is, firstly, put the polishing metallo-graphic samples into 8% nitric acid alcohol solution tocorrode, then evaporate it with a layer of about 20³30 nmthick carbon film. Finally, extract precipitates by using 10%nitric acid alcohol solution, and the carbon membrane isgained by copper net. Morphologies of precipitates can beobserved by using TEM after the copper net is being dried.

Deploying ZBC2452-B Pendulum impact testing machine,the impact values of three samples are tested. DeployCMT4105 electronic universal testing machine to test tensilestrength, yield strength, elongation and area reduction rate.DHB-3000 Brinell hardness (HBW2.5/187.5) tester is usedto measure hardness of samples. In addition, observeorganizations of steel by using 9XP-PC optical microscopy,and observe the morphology of carbides of three samples byusing High Resolution Transmission Electron Microscope(F30). Accelerating voltage of TEM is 300 kV.

3. Results and Discussions

3.1 Microstructure observationThe microstructures of H13 after annealing are composed

of granular pearlite, small amounts of ferrite and carbidephase. The microstructures of different positions are shown inFig. 2.

From Fig. 2, it can be seen that microstructures of middleand bottom are more uniform than those of upside, wherethere exist large eutectic carbides and element segregationand banded carbides. It is also found that there are lots ofcarbides segregated among dendrites. These carbides becomethe source of microsegregation during solidification proc-ess.14) The enrichment of large amounts of metallic elementsamong dendrites is the direct reason of formation of bandedsegregation after casting.2) The different degrees of segrega-tion of upside, middle and bottom are owing to differentsolidification speed during ESR. From upside to bottom ofH13 ingot, solidification speed increases so that degree ofupside segregation is more serious than that of bottom’s.

3.2 Mechanical propertiesFrom mechanical property of Table 2, it can be seen that

the bottom of H13 ingot after annealing has the optimalcomprehensive mechanical property. It sets solid foundationon the consequent quenching and tempering process. Thechange of elongation and hardness show that the middle ofH13 has the best ductility. From upside to bottom, the changeof property is as follows, tensile strength and yield strengthboth have tendency of increasing, instructing that strengthincreases as well as plasticity becomes better. The increase ofimpact value indicates that the toughness improves severelyfrom upside to bottom of H13 ingot. In conclusion, thebottom of H13 ingot has the best comprehensive mechanicalproperty.

3.3 The morphologies and types of H13 precipitatesThrough electron diffraction and EDS (Energy Dispersive

Spectrometer) analysis, it can be concluded that precipitatetypes after annealing are mainly M23C6, V8C7 and VC, themorphologies of which are shown in Fig. 3, Fig. 4 and Fig. 5.

12R

Fig. 1 The position of sampling.

Table 1 Chemical compositions of experimental H13 steel (%).

C Si Mn P S Cr Ni Cu Mo V Al N T.[O]

0.39 0.88 0.34 0.0064 0.0005 5.13 0.086 0.054 1.5 0.99 0.047 0.0093 0.0017

upside middle bottom

Fig. 2 Microstructure organizations of H13 of different positions after annealing.

N. Angang, G. Hanjie, C. Xichun and W. Mingbo582

From Fig. 3, 4 and 5, it can be seen that spherical particlesare Cr-rich M23C6 carbides. This kind of carbides are morethan 200 nm; The shapes of V8C7 are 100 © 50 nm short bar-like. VC is square-like carbides, smaller than 100 nm.

Due to short-time stay in ¤-Ferrites15) and high speed ofsolidification during ESR, the author consider that H13directly enters austenite area after solidification. The austenitearea is between solid line 1755K17) and Ar1 line 1048K.18)

The solubility product of Cr23C6 in austenite is deduced asfollows.

Firstly, the Gibbs free energy of formation of Cr23C6 inaustenite is obtained through following chemical reaction.19,20)

23CrðsÞ þ 6CðsÞ ¼ Cr23C6ðsÞ�Gª ¼ �309600� 77:4T ð1Þ

CrðsÞ ¼ ½Cr� �Gº ¼ 19250� 46:86T ð2Þ½Cr�L ¼ ½Cr�¤ �Gº � 1046 J=mol ð3Þ½Cr�¤ ¼ ½Cr�£ �Gº � �418 J=mol ð4Þ

According to formula (2)³(4), the Gibbs free energy ofsolid Cr dissolved in austenite is obtained as Formula (5).

CrðsÞ ¼ ½Cr�£ �Gº ¼ 19878� 46:86T ð5ÞThen, according to equilibrium solubility product formula

of graphite dissolved in austenite,21)

lg½C�£ ¼ 1:595� 1762

Tð6Þ

The Gibbs free energy of graphite dissolved in austenite isobtained as follows.

(b) (c)(a)

Fig. 3 The precipitates VC in H13 steel after annealing, (a) TEM morphology, (b) SAED, (c) EDS.

(c)(a) (b)

Fig. 4 The precipitates V8C7 in H13 steel after annealing, (a) TEM morphology, (b) SAED, (c) EDS.

(b) (a) (c)

Fig. 5 The precipitates Cr23C6 in H13 steel after annealing, (a) TEM morphology, (b) SAED, (c) EDS.

Table 2 Mechanical properties of H13 after annealing.

Samplenumber

TensileStrength,Rm/MPa

Yieldstrength,Rp/MPa

Elongation,A/%

HBImpactvalue,Ak/J

1 605.72 302.62 27.84 175 13.3

2 620.33 317.53 24.64 170 33.7

3 627.17 316.01 23.44 173 59.6

Precipitation Behaviors and Strengthening of Carbides in H13 Steel during Annealing 583

CðsÞ ¼ ½C�£ �Gº ¼ 33735� 30:532T ð7ÞThe formation reaction of Cr23C6 in austenite is calculated

by using Formula (1)³(7). The metallic elements and Carbonare deployed 1% solution as standard state.

23½Cr�£ þ 6½C�£ ¼ Cr23C6ðsÞ�Gº ¼ �969204þ 1183:512T ð8Þ

Finally, the solubility product of Cr23C6 dissolved inaustenite is obtained as follows according to Formula (8).

lnðw½Cr�23% � w½C�6%Þ£ ¼ 142:35� 116574:93

Tð9Þ

Formula (9) can be converted to Formula (10).

lgðw½Cr�23% � w½C�6%Þ£ ¼ 61:81� 50618:73

Tð10Þ

According to literature,21) the solubility products of VCdissolved in austenite and V8C7 dissolved in ferrite are asfollows,

lgðw½V�% � w½C�%Þ£ ¼ 6:72� 9500

Tð11Þ

lgðw½V�% � w½C�0:875% Þ¡ ¼ 5:65� 9340

Tð12Þ

Put w½Cr�% ¼ 5:13, w½V�% ¼ 0:99, w½C�% ¼ 0:39 intoformula (10), (11) and (12), and the precipitation temper-atures of V8C7, VC and Cr23C6 are calculated as 1553.8K,1331.8K and 1056.0K respectively.

From Fig. 6, when lowering H13 from austenite temper-ature to room temperature, the solubility product of Cr23C6

decreases rapidly. Although precipitation temperature ofCr23C6 is lower than that of VC and V8C7, Its precipitationamount is more than that of V8C7 and VC.

The precipitation sequence of three types of carbides isV8C7 > VC > Cr23C6. It can be concluded that V8C7 startsto precipitate during ESR process before forging, while VCand Cr23C6 start to precipitate during forging process andannealing. As annealing for 15 hours at 1133K, it can be seenthat V carbides do not have obvious tendency of growing up,while Cr carbides grow up obviously during annealing.

3.4 The statistics of precipitate amounts and calculationof precipitation strengthening

Figure 7 shows the precipitate morphologies of threepositions in H13. It can be seen that the sizes of precipitateparticles are large, and distribute unevenly. The amounts ofprecipitates become larger from upside to bottom of ingot.The distribution and size of precipitates have main effect onfinal mechanical properties of H13.

In order to obtain precipitate distribution accurately, 45pieces of pictures of 58.8 µm2 (large visual field) and 45pieces of pictures of 14.5 µm2 (small visual field) are takeninto statistics in this experiment. The result is shown inTable 3.

From Fig. 7 and Table 3, it can be seen that the amountof precipitates increases and average particle size becomessmaller from upside to bottom of H13 ingot. The main reasonlies in the different solidification speed during ESR process.The bottom of ingot has the highest solidification speed.Combined with Fig. 2, it can be concluded that the morerapid the solidification is, the microstructures of steel aremore even, and the precipitates distribute more dispersively.Therefore, the comprehensive mechanical property is greatlyimproved.

This article deploys the methods of McCall-Boyd22) tomeasure volume fraction of dispersive second phase. Thismethods is required to measure diameter of each precipitatein a certain area of photos taken by TEM. Then, theequivalent average diameters are available. In addition, thenumber of precipitates must be calculated.

The volume fraction of precipitates can be calculated asfollows

f ¼ 1:4³

6

� �� ND2

mean

A

� �ð13Þ

Where A is area of measured photos; Dmean is equivalentaverage diameter. According to Theory of Gladman,16) this

200 400 600 800 1000 1200 1400 1600 1800-80

-60

-40

-20

0

20

40

V8C

7

V8C

7

Cr23

C6

Cr23

C6

VC

Log

arith

m o

f So

lubi

lity

Prod

uct

Actual Solubility ProductEquilibrium Solubility Product

VC

Temperature, T/K

Fig. 6 The curve of equilibrium solubility product and actual solubilityproduct with temperature changing.

(a) (b) (c)

Fig. 7 The precipitate morphologies of H13 after annealing, (a) upside, (b) middle, (c) bottom.

N. Angang, G. Hanjie, C. Xichun and W. Mingbo584

article adopts following formula21) to calculate increment ofprecipitation strengthening of H13 after annealing.

�·p ¼ 0:8995� 104f

12

dlnð2:417dÞ ð14Þ

Where d is equivalent average diameter of precipitates,nm; f represents volume fraction of precipitates.

According to increment of precipitation strengthening ofdifferent size ranges proposed by Yong Qilong,23) the totalincrement of precipitation strengthening can be summed upby increment of precipitation strengthening of different sizeranges. The result is shown in Table 4.

From Table 5, it can be concluded that the size ofprecipitates decreases and volume fraction increases fromupside to bottom of ingot. The contributions to yield strengthof different positions are also improved gradually. Combinedwith experiment results of Table 2, it can be seen that thestrength and toughness of H13 are both improved as

precipitates are getting finer and dispersive. The middle ofingot has the best ductility, but the bottom of ingot hasthe worst. It is explained that excessive precipitates arenot benefit for ductility of H13. Therefore, in the actualproduction process, through adjusting ESR, forging and

Table 5 Contribution of precipitating strengthening to yield strength ofprecipitates.

Sample position upside middle bottom

Average size, D/nm 265.91 230.42 225.88

Volume fraction, f/% 8.11 11.90 12.26

Yield strength increment, ¸P/MPa 191.77 218.99 244.20

Actual yield strength, ·P/MPa 302.62 317.53 316.01

Proportion of precipitationstrengthening accounted

for actual yield strength/%63.37 68.97 77.28

Table 4 Contribution of precipitate strengthening to size and volume fraction of precipitates.

Size, D/nmAmount Average diameter, D/nm Volume fraction, f

Yield strengthincrement, ¸P/MPa

1# 2# 3# 1# 2# 3# 1# 2# 3# 1# 2# 3#

<50 1 17 10 28.39 26.85 29.67 0.000001 0.00001 0.00001 0.98 3.99 3.14

50³100 59 93 196 85.80 86.55 86.12 0.000289 0.00046 0.00097 9.51 11.96 17.35

100³150 268 370 741 128.20 127.30 125.33 0.002935 0.00400 0.00776 21.80 25.59 36.11

150³200 266 318 501 174.45 172.53 170.83 0.005394 0.00631 0.00974 22.89 24.98 31.30

200³250 161 190 207 220.63 224.82 223.33 0.005222 0.00640 0.00688 18.50 20.16 21.02

250³300 91 88 100 271.66 272.55 271.55 0.004475 0.00436 0.00491 14.37 14.14 15.06

300³350 42 55 57 327.06 325.36 324.08 0.002994 0.00388 0.00399 10.04 11.48 11.68

350³400 44 41 59 373.17 377.69 375.48 0.004083 0.00390 0.00554 10.48 10.13 12.15

400³450 39 36 48 425.71 425.10 425.72 0.004710 0.00434 0.00580 10.06 9.66 11.16

450³500 34 43 34 473.65 470.71 471.24 0.005083 0.00635 0.00503 9.54 10.71 9.53

500³550 27 29 28 524.88 528.32 529.06 0.004957 0.00539 0.00522 8.62 8.94 8.79

550³600 24 25 29 576.39 576.75 576.85 0.005313 0.00554 0.00643 8.23 8.41 9.05

600³650 15 18 20 624.83 624.44 620.47 0.003902 0.00468 0.00513 6.58 7.21 7.59

650³700 19 22 22 675.31 672.78 676.2 0.005774 0.00664 0.00670 7.49 8.05 8.06

700³750 8 15 23 723.30 719.56 726.99 0.002789 0.00518 0.00810 4.90 6.71 8.32

750³800 14 15 17 770.42 776.21 778.21 0.005537 0.00602 0.00686 6.54 6.78 7.22

800³850 12 11 13 823.97 826.44 826.31 0.005429 0.00501 0.00591 6.11 5.85 6.36

850³900 3 10 3 867.37 870.22 881.31 0.001504 0.00505 0.00155 3.08 5.62 3.08

900³950 4 5 6 923.40 924.24 922.6 0.002273 0.00285 0.00340 3.58 4.00 4.39

950³1000 4 7 7 966.79 972.03 972.03 0.002491 0.00441 0.00441 3.60 4.77 4.77

>1000 7 28 19 1124.85 1235.26 1199.01 0.005902 0.02847 0.01820 4.86 9.83 8.07

Total 1142 1436 2140 0.081058 0.119 0.12255 191.77 218.99 244.20

Table 3 The statistics of amounts and average sizes of upside, middle and bottom of H13.

SampleVisual field area,

A/µm2The amountof photos

The amountof precipitates

The total amountof precipitates

The amount of precipitatesper area/µm¹2

Average size,D/nm

Total average size,D/nm

1/2 radiusof upside

58.8 15 841 1142 1.04 278.83 265.91

14.5 15 301 229.80

1/2 radiusof middle

58.8 15 1017 1521 1.38 252.81 230.42

14.5 15 504 185.23

1/2 radiusof bottom

58.8 15 1366 2140 1.95 253.47 225.88

14.5 15 774 177.19

Precipitation Behaviors and Strengthening of Carbides in H13 Steel during Annealing 585

annealing processes, i.e. adjusting cooling rate or ratio ofalloying elements, based on obtaining uniform organization,control the amounts of precipitates in order to obtain H13steel with optimal property.

4. Conclusion

(1) The types of precipitates of H13 are mainly Cr-richM23C6 and V-rich MC, the size of which are about200 nm and 100 nm. The shapes are sphere-like andsquare-like. The precipitation temperatures of V8C7,VC and Cr23C6 are 1553.8K, 1331.8K and 1056.0Krespectively through thermodynamic calculations.

(2) From upside to bottom of H13 ingot after annealing,precipitates are getting finer, volume fraction increasesand contribution to yield strength is improved.

(3) The contribution to yield strength of precipitates is291³344MPa, which takes up 63³77%. As precipitateamount increases and particle size becomes finer, thecomprehensive mechanical properties are improved.The bottom of H13 ingot has optimal mechanicalproperty. As large size and amount of carbidesprecipitating during annealing, too many precipitatesare not benefit for ductility of H13, but has little effecton toughness and strength.

Acknowledgments

The authors acknowledge that this research is supported bythe state foundation for key projects: Investigation on nano-scale precipitates in hot work die steel and comprehensivestrengthening mechanism of steel (No. 51274031).

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