The effect of heat treatment on microstructure and properties of laser-deposited TiC reinforced H13 steel matrix composites

Journal Pre-proof The effect of heat treatment on microstructure and properties of laser-deposited TiC reinforced H13 steel matrix composites Minglang Zhang, Cong Li, Qiong Gao, Jimi Fang, Tong Li Wu, Ke Hong Wang

PII:

S0030-4026(20)30120-0

DOI:

https://doi.org/10.1016/j.ijleo.2020.164286

Reference:

IJLEO 164286

To appear in:

Optik

Received Date:

10 December 2019

Accepted Date:

21 January 2020

Please cite this article as: Zhang M, Li C, Gao Q, Fang J, Wu TL, Wang KH, The effect of heat treatment on microstructure and properties of laser-deposited TiC reinforced H13 steel matrix composites, Optik (2020), doi: https://doi.org/10.1016/j.ijleo.2020.164286

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The effect of heat treatment on microstructure and properties of laser-deposited TiC reinforced H13 steel matrix composites Minglang Zhang, Ke Hong Wang xiaolingwei200

Abstract

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With the development of laser technology, laser processing technology has become increasingly popular in the additive manufacturing, remanufacturing and repairing of hot work molds. Due to the special nature of laser processing technology, it is obviously different from the traditional

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forging or casting methods, resulting in the need for re-exploration and formulation of the heat

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treatment process. Therefore, this paper hopes to use laser deposition technology to add hard

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ceramic phase to prepare H13 steel composites, and to explore the effect of tempering treatment on the microstructure and properties of TiC reinforced H13 steel composites, and to provide

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theoretical support and practical reference for extending the life of H13 steel. The results indicated that the structure after laser deposition is mainly martensite and fine carbides. With the increase of

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the proportion of TiC, the structure of the sample becomes more fine and uniform. After tempering,the microstructure of the sample is high-temperature tempered martensite and fine

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carbides,and the distribution of TiC is more uniform.Compared with untempered amples ,the hardness and tensile strength of the sample tempered are lower, but the wear resistance is enhanced. Keywords: H13 steel;TiC;laser deposition;microstructures;Mechanical ;tempering

1. Introduction

Mould is an extremely important industrial equipment in modern manufacturing, and it has become an important indicator to measure a country's manufacturing level [1]. The level of the mold industry is directly reflected in the quality and life of the mold. Hot work die steel is used for hammer forging, warm forging, die casting, hot extrusion, etc. Improving the quality and life of the mold is equivalent to reducing costs and expanding profit margins. Therefore, the industrial community urgently needs to seek new processing methods to promote the transformation and

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upgrading of mold manufacturing [2,3].

Laser additive manufacturing technology [4,5] uses laser as a heat source, which has lower heat input than other heat sources, and has the characteristics of short reaction time with the material

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and fast solidification speed, which makes the shaped sample have a smaller heat-affected zone

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and pole. Small thermal distortion. Laser additive manufacturing technology has successfully used

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the interaction between laser and powder and has been widely used in the remanufacturing of molds. The ultimate goal put forward by the manufacturing industry is to directly and accurately

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manufacture metal parts, components and even assembled metals. Artifact products. The heat treatment process has the functions of adjusting the microstructure and improving the

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mechanical properties. Therefore, the heat treatment process is applied to the laser deposition to form a part. Due to the special nature of the laser deposition manufacturing technology, the

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structure is significantly different from the traditional forging or casting methods, so that the heat treatment process needs to be explored and formulated again[6,7]. At the same time, there is relatively little research on the research of ceramic particle reinforced hot work die steel at this stage. Therefore, this project hopes to deeply analyze the laser additive manufacturing process and workpiece by studying the effect of tempering on hot work die steel

components with different proportions of ceramic particles. The internal relationship between the microstructure and thermal fatigue properties lays an experimental and theoretical basis for the actual production of ceramic particle-reinforced hot work molds for complex laser additive manufacturing.

2. Experimental details

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The schematic diagram of the experimental process is shown in Fig. 1. The H13 steel and TiC powders are ball-milled and mixed, and the obtained mixed powders of different TiC ratios are

sent to a laser processing system through a powder feeder and deposited on the H13 steel base

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layer by layer. The microstructure and properties of the deposited samples were studied after

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tempering, and finally experimental data were obtained to analyze the effect of tempering on the

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microstructure and properties of TiC reinforced H13 steel matrix composites with different

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proportions.

Fig. 1. Experimental flow diagram

The original powder used for laser deposition was a mixed powder of TiC and H13. The original particle size of H13 powder is about 50 ~ 100μm (see Fig. 2(a)), and the original particle size of TiC powder is about 3 ~ 5μm (see Fig. 2(b)).The chemical composition of H13 steel and TiC are shown in Table 1 and Table 2 respectively.Before the experiment, the H13 steel base was

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sanded, and the surface of the substrate was cleaned with acetone to remove surface oil.

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Fig. 2. Microstructure of the original powder

Table 1

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Chemical compositions of H13 steel (Wt,%)

Si

Mn

Cr

Mo

V

P

S

0.32-0.45

0.80-1.20

0.20-0.50

4.74-5.50

1.10-1.75

0.80-1.20

≤0.030

≤0.03

Table 2

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C

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Chemical compositions of TiC (Wt,%)

Total carbon content

Free carbon

Nb

Fe

Si

O

N

≥19.10

≤0.30

≤0.01

≤0.05

≤0.02

≤0.50

≤0.20

Considering the impact of different densities of H13 steel powder (8.9g/cm3) and TiC powder (4.09g/cm3) on the quality of the composite material deposited, before the deposition test, vertical

ball mills and horizontal ball mills were used to mix different proportions of H13 steel powder and TiC powder by mechanical alloying.Then a better mixed powder for laser deposition was choosed. Among them, the proportion of TiC is 10%, 20% and 30%, respectively. The vertical ball mill ball-to-material ratio is 5:1, and the powder is filled in four cans (each can 50g) at a time. The parameters have been adopted as shown in Table 3.The ball-to-material ratio is 20:1,and 200g powder is loaded each time when horizontal ball mill was used whose ball milling spindle speed is

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180r/min time is 35min.

Fig. 3 and Fig. 4 shows the results of the two methods of ball milling. when the vertical ball mill was selected (see Fig. 3), TiC and H13 did not achieve a good combination. Since then,

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different parameters shown in Table 3 have been used, but none have achieved good results. The

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reason is that TiC particles are unevenly distributed and cannot be embedded in H13 particles

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during ball milling. Due to the different densities of H13 and TiC powders, the two particles fall in a different order during powder feeding and laser deposition, and cannot achieve a good

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deposition effect.

When the horizontal ball mill was used for ball milling, the ball milling results are shown in Fig.

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4. One can see that the surface of the particles is uneven and the particle in Fig 4(a) is enlarged as shown in Fig. 4(b).According to the EDS analysis of the microscopic magnification in Fig. 4(c)

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after point scanning, it was found that the H13 particles were embedded in TiC. Therefore, when the mixed powder was obtained by the horizontal ball milling mechanism, TiC powder particles are uniformly embedded in the H13 powder particles, which have a better combination than the previous mixed powder obtained by vertical ball mill (see Fig. 3) and overcomes the disadvantages of different density of H13 and TiC powders during feeding powder. Therefore, the

subsequent preparation of mixed powder experiments uses a horizontal ball mill. Table 3 Vertical ball spindle paraments

250r/min

250r/min

300r/min

300r/min

tinme

2.5h

23h

2.5h

3h

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spindle speed

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Fig. 3. Morphology of mixed powder produced by vertical ball

Fig. 4. Morphology of mixed powder produced by horizontal ball mill

A laser-milled TiC and H13 mixed powder was deposited on a H13 steel substrate using a laser. The laser deposition equipment uses TRUMPF disc lasers. The laser deposition process parameters are shown in Table 4. The actual process of laser processing is shown in Fig. 5.

Table 4 Laser deposition process parameters

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Fig. 5. Actual laser processing process

Scan speed

Amount of powder

Powder feeding speed

2600W

8mm/s

24g/min

1.2r/min

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Energy

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The laser-deposited sample is processed to prepare a metallographic sample whose size is 10mm × 10mm and thickness is finally 2.0 ± 0.1mm. The metallographic etchant used in this

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paper is a mixed solution of nitric acid and alcohol, and the corrosion time is 25-30s. The sample was put into a muffle furnace for tempering. Two times tempering processes are used. Secondary

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Tempering process was adopted.Table 5 shows the tempering process parameters.

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The microhardness test was performed on the untempered samples and the tempered samples. Test parameters: load 100g, holding time 10s.

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Abrasive abrasion tests were performed on untempered abrasion specimens and abraded specimens after tempering. Under the same model of sandpaper and abrasion time, the weight loss is used as the comparison standard. The samples before and after abrasion were cleaned and dried with absolute ethanol in an ultrasonic cleaning tank, and the calculated mass difference was the weight loss. The load is 20N and the wear time is 10min.

Table 5

Tempering process parameters Process parameters

Temperature

Holding time

Cooling options

Tempering once Tempered twice

600℃ 650℃

1h 2h

air cooling air cooling

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3. Results and discussion

3.1Effect of Tempering on Typical Morphology of TiC H13 Matrix Composites

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Fig. 6 shows the macroscopic morphology of laser-deposited TiC reinforced H13 steel matrix

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delamination and a good deposition effect.

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composites with different proportions. It can be seen that the laser deposition has obvious

Fig. 6. Macroscopic morphology of laser deposition

Observing the cross-section morphology of samples (untempered) containing different proportions of TiC under a laser confocal scanning microscope (see Fig. 7), the sample cross section can be divided into columnar grain region and equiaxed grain region from top to bottom. It

can be seen that there are no defects such as holes and cracks in the bonding area. The grains in each layer grow into columnar crystals roughly along the direction of heat flow; between the layers, due to the heat accumulation in the solidification zone during the laser deposition process and the remelting of the front end ,the temperature gradient decreases and the undercooling degree increases to grow into equiaxed grains. In addition, with the increase of the proportion of TiC, the grains of columnar grains and equiaxed grains gradually become smaller , more blunt and

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uniform.

Fig. 7. Typical section morphology:(a)0%TiC,(b)10%TiC,(c)20%TiC,(d)30%TiC

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Fig. 8 shows a typical cross-sectional morphology of a 20% TiC sample after tempering.

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Compared with the 20% TiC sample without tempering treatment, the cross-section grain distribution of the sample after tempering treatment is more uniform and finer. The reason for this phenomenon is that the dispersed distribution of TiC during tempering hinders the grain boundary migration and makes the grains more fine and uniform [8].

Fig. 8. Typical section morphology of sample tempered with 20% TiC

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3.2 XRD phase analysis

X-ray diffractometer was used to perform phase analysis on the deposited samples. The results

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are shown in Fig. 9. It can be seen that the pure H13 steel (0%TiC) deposited samples mainly consisted of α-Fe solid solution, and the samples with TiC content of 10% to 30% are mainly

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composed of TiC and α-Fe solid solution, that is, martensite phase. The phase composition of the

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diffraction peaks of the tempered sample and the untempered sample is basically the same.In

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addition, some phases have very little content without detection and no diffraction peaks.

Fig. 9. XRD images of different proportions of TiC samples

3.3 Effect of tempering on microstructure of TiC H13 composites

Fig. 10 shows the microstructure of TiC samples with different proportions without tempering. The microstructure of the deposited sample is "Fine martensite + undissolved carbide + small amount of retained austenite" [9]. It can be seen that the structure of pure H13 steel is relatively coarse and unevenly distributed.

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With the increase of the proportion of TiC, the structure becomes fine and uniform, and the TiC particles have a discontinuous chain structure in the matrix.The reason for the larger particles is

that the density of TiC is smaller. When subjected to buoyancy and convection, the partially

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dissolved TiC particles collide to form larger particles[10].

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Fig. 10. Microstructure of samples untempered:(a)0%TiC,(b)10%TiC,(c)20%TiC,(d)30%TiC

Fig. 11 shows the microstructure of samples with different proportions of TiC after tempering. It

can be seen that the specimens after tempering precipitated fine carbides, and the structure which was transformed into high-temperature tempered martensite is finer and more uniform than untempered samples. This trend become even more obvious as the proportion of TiC increases.

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Fig. 11. Microstructure of specimens tempered:(a)0%TiC,(b)10%TiC,(c)20%TiC,(d)30%TiC

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3.4 Effect of Tempering on Microhardness of TiC H13 Composites

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Fig. 12 is a hardness comparison of the tempered sample and the untempered sample. For untempered laser-deposited samples,the hardness of samples containing 0% -30% TiC is 684.2HV,

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713.5HV, 750.6HV, 784.7HV, respectively. It shows that the hardness of the sample is increased due to the addition of TiC, and the hardness of the deposited layer increases with the increase of

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the TiC content. This is due to the distribution of cemented carbide TiC in the deposited layer. On

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the one hand, it is due to the dispersed distribution of TiC particles, which acts as a diffusion strengthening to prevent dislocation movement; on the other hand, it is because the TiC particles are embedded in the grain boundaries, effectively Preventing the growth of crystal grains during the deposition process has the effect of strengthening the fine grains, thereby increasing the hardness of the sample [11]. After tempering, the hardness of the samples containing 0% to 30% of TiC is 510.3HV,

530.6HV, 560.8HV, 580.4HV in order.Compared the microhardness of tempered and untempered samples,tempering reduces the hardness of the corresponding proportion of samples. The reason is that the tempering treatment transforms the retained austenite and supersaturated martensite formed by laser processing to tempered martensite and carbides, reducing residual stress, accelerating the diffusion rate of atoms, rearranging dislocation packets, and reducing vacancies. The dislocation density decreases, thereby reducing the hardness of the sample. In addition,

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studies have shown that when the tempering temperature is 650 ℃, the matrix will recover, and the carbides that originally precipitated out of the grain boundaries will grow into a discontinuous chain structure and loss of the coherent relationship with the matrix. In this case, carbon atoms are

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precipitated and the matrix becomes soft.Therefore, the hardness of the substrate is greatly

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reduced [12].

Fig. 12. Microhardness of untempered and tempered samples

3.5 Effect of Tempering on Wear Resistance of TiC H13 Steel Composites

The comparison of the weight loss between the untempered and tempered samples is shown in

Fig. 13. It is observed that that the weight loss of wear becomes smaller as the proportion of TiC increases which is own to the high hardness of TiC[13],So the addition of TiC can improve the wear resistance of H13 steel. The abrasion resistance of the samples after tempering still increased with the increase of TiC ratio. For the tempered samples containing the same proportion of TiC, the weight loss is much smaller than that the untempered samples. Therefore the abrasion resistance of the samples after

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tempering is enhanced. This is because the high-temperature tempered martensite structure and fine carbides formed after tempering can effectively block wear and reduce the coefficient of

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friction.

Fig. 13. Grinding loss of untempered and tempered samples

3.6 Effect of Tempering on tensile strength of TiC H13 Steel Composites

The tensile strength-strain curves of tempered and untempered composites at various TiC ratios

are shown in Fig. 14 . The specific values are listed in Tables 6 and Table 7. One can see that the tensile strength of the composite becomes lower and the strain increases after tempering. The main reason is that a large amount of fine carbides are dispersed and precipitated in the composite material after laser deposition. When the tempering temperature is higher than 650 °C, the dislocation density in the martensite will be further reduced and the precipitated carbides will grow. Therefore, the substrate is softened, and finally the tensile strength is reduced. The strain

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changing trend of composites tempered is opposite to the tensile strength.

Fig. 14. Stress strain curve of TiC/H13 composites untempered and tempered

Table 6 Stress-strain of TiC/H13 steel matrix composites untempered

Proportion of TiC

0%TiC

10%TiC

20%TiC

30%TiC

Tensile strength(MPa)

1180

1380

1550

1610

Strain

4.8%

7.4%

8.4%

9.1%

30%

Table 7 Stress-strain of TiC / H13 steel matrix composites tempered

0%

10%

20%

Tensile strength(MPa) Strain

950 5.4%

1140 8.4%

1290 9.6%

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Proportion of TiC

1395 10.2%

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3.7 Fracture morphology of TiC / H13 steel matrix composite before and after tempering

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Fig. 15 shows the typical micro-fracture morphology of TiC-reinforced H13 steel matrix composite tempered and untempered.Fig. 15(a) shows the tensile morphology of H13 steel with

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0% TiC content. It can be clearly seen that there are cleavage steps and river patterns, these are the

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basic microscopic characteristics of cleavage faults. Cleavage fracture belongs to brittle fracture, so it can be judged that the fracture form of the laser-deposited sample of pure H13 includes brittle

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fracture. In addition, there are a small number of circular and oval dimples of different sizes in the figure. This is the characteristic of ductile fracture, so the fracture form also includes ductile

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fracture. Therefore, the fracture form of H13 steel without TiC is tough and brittle mixed fracture. According to Fig. 15(b) shows the tensile fracture morphology of TiC/H13 steel matrix

composite with 30% TiC. We can see a large number of round and oval dimples of different sizes. According to the distribution of the dimples, it can be concluded that it is an isometric dimple. The size of the dimples is much larger than the dimples without TiC, with a larger number and a more

even and denser distribution. Due to the high fracture toughness and low density of the second phase particle TiC, and the strain hardening index value of the TiC/H13 powder after mixing is reduced (the low strain hardening index affects the connection and polymerization of micropores),the material is more prone to internal necking during tensile fracture, which increases the size and number of micropores. Therefore, the toughness of TiC/H13 steel matrix composites is improved [14].Therefore, the fracture form of TiC / H13 steel matrix composite with 30% TiC is

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ductile fracture.

While the same proportion of TiC / H13 composite material, only the dimples were observed after the sample was tempered, and the dimples became larger(see Fig. 15(c) and Fig.

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15(d)).Therefore, after tempering, the sample is ductile fracture.The tensile strength of the

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composite material decreases,and the strain increases after tempering.

Fig. 15. Typical fracture morphology (a) 0%TiC untempered (b) 30%TiC untempered

(c) 0%TiC tempered (d) 30%TiC tempered

3 Conclusion

In this paper, TiC reinforced H13 steel matrix composites were prepared by laser deposition technology, and the effect of tempering treatment on the microstructure and properties of the composites was studied. The full text can be concluded as follows:

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(1) The microstructure of the composite material after laser deposition is fine and uniform with

the increase of TiC. The structure before tempering is mainly martensite and fine carbide structure. After tempering at 650 ℃, the structure is high temperature tempered martensite and precipitated.

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Carbide, the structure is more uniform than before tempering.

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(2) As the proportion of TiC increases, the hardness of the composite material increases and the

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wear resistance increases. The hardness of the tempered sample is lower than that of the untempered sample, but the wear resistance is enhanced.

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(3) With the increase of TiC, the tensile strength and elongation of the composites increase; the shape of the micro-fractured dimples becomes larger and the number increases, and the fracture

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form changes from ductile-brittle mixed fracture to ductile fracture. After tempering, the tensile strength of the composite material decreases, the elongation increases, and the fracture form is

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ductile fracture.

Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work.We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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