Effect of carbon content on the microstructure and the cracking susceptibility of Fe-based...

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Applied Surface Science 240 (2005) 63–70

Effect of carbon content on the microstructure and the cracking

susceptibility of Fe-based laser-clad layer

Sheng Li*, Qian-Wu Hu, Xiao-Yan Zeng, Sheng-Qin Ji

State Key Laboratory of Laser Technology, Huazhong University of Science and Technology,

Wuhan, Hubei 430074, PR China

Received in revised form 2 June 2004; accepted 2 June 2004

Available online 19 August 2004

Abstract

The laser cladding of Fe-based alloys on a medium carbon steel substrate was performed using a CO2 laser and Ar shielding

gas that was blown into a molten pool. The microstructure and cracking susceptibility of the laser-clad layers were studied in

terms of carbon additions. Results show that the small change of the carbon content in the alloy powders can obviously change

the microstructure and properties of the layers. When the carbon content is in the range of 0.3–0.4 wt.%, the decrease of the

carbon content in alloy powders will increase the hardness and toughness of the layers simultaneously under the same process

parameters. As a result, crack-free coatings with high hardness can be obtained. As the carbon content increases from 0.2 wt.%

to 0.4 wt.%, the segregation ratio of chromium increases, while the segregation ratios of nickel, manganese, and silicon first

decrease and then increase. At the same time, a new designing principle concerning the composition and microstructure has been

put forward, and the principal mechanisms of strengthening and toughening of the layers are fine-grain strengthening and low

carbon martensitic phase transformation strengthening.

# 2004 Elsevier B.V. All rights reserved.

PACS: 42.62.�b; 61.82.Bg

Keywords: Laser cladding; Fe-based alloy; Carbon content; Cracking susceptibility; Peritectic

1. Introduction

Compared with other conventional surface treat-

ment techniques such as flame spraying, plating, and

arc welding, laser cladding has several advantages

* Corresponding author. Tel.: +86 27 87541780;

fax: +86 27 87541423.

E-mail address: [email protected] (S. Li).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2004.06.098

attracting researchers to pay much attention to its

application in industry. (1) Laser cladding creates a

true metallurgical bond between the cladding layers

and the base material. (2) The heating and cooling

rates are high, resulting in a fine microstructure and/or

metastable phases. (3) The dilution and the heat-

affected zone of the base material are very small.

(4) Local treatment is possible. However, due to the

very high cooling rate of layers, laser-clad layers are

.

S. Li et al. / Applied Surface Science 240 (2005) 63–7064

easy to crack, especially when the hardness of layers is

high. As a result, this technology is not widely used in

factories. In order to solve this problem, material

researchers have put up with several measures such

as preheating, tempering, optimizing process para-

meters and using graded coatings [1,2]. But these

measures have some drawbacks, such as increasing

the cost of production, reducing the efficiency of

production and worsening the working conditions.

Furthermore, the effects of these measures are very

limited [3,4]. Hence, the crack problem does not

obtain a very good settlement.

We think that the best approach to decreasing the

cracking susceptibility is to study new cladding mate-

rials suited to the technology. In fact, there have

already been some reports on the application of this

approach to laser cladding. It has been pointed out that

increasing Ni and Co content can improve the tough-

ness of the layers and decrease the cracking suscept-

ibility [4–7]. However, adding large amounts of

toughness phase elements, such as Ni and Co, to

alloys will bring several harmful effects, such as

lowering the hardness and prosperities of the layers

and increasing the cost of production. It is well known

that carbon plays a very important role on the micro-

structure and property of Fe-based alloy, but there is

little data available regarding the effects of carbon

content on the microstructure and cracking suscept-

ibility of laser-clad layers. In this paper, we have

investigated them with several kinds of Fe-based alloy

powders, which contain different carbon content, and

have drawn some significant conclusions. We hope

that our results will be beneficial to the design of laser-

clad alloys.

Fig. 1. Effect of C content in the powders on the hardness of clad

layer.

2. Experimental procedures

Fe-based alloy powders self-made (Fe0) with var-

ious carbon contents were laser clad onto medium

carbon steel substrates of Ø 40 mm � 10 mm with

argon gas shielding (blowing to melting pool in the

open system). The alloy powders Fe0 had the compo-

sition (wt.%) Fe95.5–Ni1.5–Cr1–Mn1–Si1.

The laser used was a continuous wave (CW) CO2

type operated at 3.5 kW, 4 mm laser beam diameter

(D). The thickness of the preplaced powder was

1.2 mm. The traverse speed Vs was 6 mm/s and

3 mm/s. The overlapping number was 8 and the over-

lapping factor was 0.33.

The cracking susceptibility of the layers was

assessed via the length of crack: (a) in unit cladding

area (cm2)

a ¼P

Li

A

Li (mm) is the length of crack I and A the area of clad

layer. The penetration visualization method was used

to measure cracks. A microscope at 10� was used to

measure the crack number of the layer.

The macro-hardness tests of the cladding layers

were performed using a Rockwell type tester, and the

micro-hardness tests were performed using a Vickers-

1000 digital micro-hardness tester with a load of

0.098 N and a dwelling time of 15 s. Microstructures

were studied by means of optical microscopy (OM)

and scanning electron microscopy (SEM), and X-ray

diffraction structures analysis was carried out. The

average chemical compositions of the phases were

analyzed by energy dispersive spectroscopy (EDS)

and the average chemical compositions of the layers

were studied by means of chemical analysis.

3. Results

3.1. Effect of carbon content in the powders on the

hardness of clad layer

The effects of carbon content in the powders on the

hardness of clad layers are shown in Fig. 1. Under the

same traverse speed, the hardness of clad layers using

S. Li et al. / Applied Surface Science 240 (2005) 63–70 65

Fig. 2. Effect of C content in the powders on the cracking suscept-

ibility of clad layer.

the powders containing 0.3 wt.% C (Fe2) is higher than

that of clad layers using the powders containing

0.2 wt.% C (Fe1) and also than that of clad layers using

the powders containing 0.4 wt.% C (Fe3). Fig. 1 also

indicates that, for all the three kinds of powders used,

the hardness of clad layers increases with increasing the

traverse speed. Apparently, this is a good method of

changing the hardness of clad layers slightly.


cracking susceptibility of clad layer

The effects of carbon content in the powders on the

cracking susceptibility of clad layers are shown in Fig.

2. It shows that, despite the change of the traverse

speed, the clad layers using Fe1 and Fe2 are crack-

free. This indicates that these two kinds of alloy

powders have good properties of resisting cracking.

Cracks can be observed in the laser-clad layers using

Fe3, though the length of cracks decreases with

decreasing the traverse speed.

Table 1

The average contents of Cr, Ni, Mn, and Si in grain boundary and in ma

Cr (at.%)

Fe1, 6 mm/s (see Fig. 4a (1)) 1.16

Fe1, 6 mm/s (see Fig. 4a (2)) 0.85

Fe2, 6 mm/s (see Fig. 4b (1)) 1.17

Fe2, 6 mm/s (see Fig. 4b (2)) 0.83

Fe3, 6 mm/s (see Fig. 4c (1)) 1.55

Fe3, 6 mm/s (see Fig. 4c (2)) 0.80

Analysis points (see Fig. 4): 1, grain boundary; 2, matrix.


microstructure of clad layer

Fig. 3 shows the effects of carbon content and the

traverse speed on the microstructures of clad layers.

Under the relatively high traverse speed (6 mm/s), with

the increase of the carbon content from 0.2 wt.% to

0.4 wt.%, the main structures of the clad layers change

from the mixed structures of black troosite and white

ferrite (Figs. 3a and 4a) to the mixed structures of

cryptocrystal martensite and ferrite (Figs. 3c and 4b),

and then to the mixed structures of plate martensite and

ferrite (Figs. 3e and 4c). While under the relatively low

traverse speed (3 mm/s), with increasing carbon con-

tent, the main structures of the clad layers change from

the mixed structures of troosite and ferrite (Fig. 3b) to

the mixed structures of martensite, black sorbite

(HV418) and ferrite (HV390) (Fig. 3d and g), and then

to the mixed structures of pearllite and ferrite (Fig. 3f).

Fig. 3 (except Fig. 3f) also indicates white pro-eutectoid

ferrite networks precipitated along prior austenite grain

boundaries. Therefore, the change law of the prior

austenite grain size can be found. Under the same

process parameters, as the carbon content increases,

the average grain size of prior austenite decreases at

first, and then increases afterwards. With decreasing the

traverse speed, the average grain size of prior austenite

increases obviously when the same alloy powder is

used. Fig. 3f shows a piece of crack in the clad layer. The

crack is black and crooked, and this reveals the crack in

the Fe-based laser clad layers is hot crack.


solute segregation ratio of clad layers

Table 1 shows the average contents of Cr, Ni, Mn,

and Si in the grain boundary and in the matrix,

trix, respectively

Ni (at.%) Mn (at.%) Si (at.%)

1.52 1.57 1.26

1.38 0.93 1.07

1.39 1.41 1.12

1.38 0.94 1.11

1.63 1.61 1.22

1.35 0.92 1.02


Fig. 3. OM micrographs of clad layers: (a) 0.2 wt.% C (Fe1), 6 mm/s; (b) 0.2 wt.% C (Fe1), 3 mm/s; (c) 0.3 wt.% C (Fe2), 6 mm/s; (d) 0.3 wt.%

C (Fe2), 3 mm/s; (e) 0.4 wt.% C (Fe3), 6 mm/s; (f) 0.4 wt.% C (Fe3), 3 mm/s; (g) 0.3 wt.% C (Fe2), 3 mm/s.

respectively. And Fig. 5 shows the calculated segrega-

tion ratios of these solute atoms. As can be seen from

Table 1 and Fig. 5, the phenomena of solute segrega-

tion are obvious although the segregation ratios vary

with solute atoms and carbon content. Fig. 5 also

shows that, as the carbon content increases from

0.2 wt.% to 0.4 wt.%, the segregation ratio of chro-

mium increase, while the segregation ratios of nickel,


Fig. 3. (Continued ).

manganese and silicon first decrease and then

increase. Previous investigation [8] has shown that

the grain size could play an important role in the

segregation phenomena, and usually increasing grain

size will increase segregation when the solute content

is small. As stated in Section 3.3, when the carbon

content increases from 0.2 wt.% to 0.4 wt.% the aver-

age grain size of layers decreases at first and then

increases afterwards. This is the most important rea-

son that the segregation ratios of most solute atoms

such as Ni, Mn, and Si are small when the carbon

content in powders is 0.3 wt.%.

3.5. X-ray diffractometry phases analysis

Fig. 6 shows that, under the relatively high

traverse speed (6 mm/s), the main phases of the

cladding layers using Fe1, Fe2, and Fe3 are all a-

Fe. Table 2 shows the theoretical lattice constant of

a-Fe and the calculated lattice constants of a-Fe

from the XRD results of clad layers. As can be seen

in Table 2, with the increase of the carbon content

from 0.2 wt.% to 0.4 wt.%, the deviations on lattice

constant increase. As a result, compared with the

Fig. 4. SEM micrographs of clad layers: (a) Fe1, 6 mm/s; (b) Fe2,

6 mm/s; (c) Fe3, 6 mm/s; 1, grain boundary; 2, matrix.Table 2

The a-Fe phase lattice constants in the cladding layers

Fe (A) 2.861

Fe0 + 0.2 wt.% C, 6 mm/s (A) 2.873

Fe0 + 0.3 wt.% C, 6 mm/s (A) 2.876

Fe0 + 0.4 wt.% C, 6 mm/s (A) 2.881

layers using Fe1 and Fe2, the layers using Fe3 have

low toughness. This is one of the most important

reasons that the cladding layers using Fe3 have high

cracking susceptibility.


Fig. 5. Solute segregation ratios. Analysis points (see Fig. 4): 1,

grain boundary; 2, matrix.

Fig. 6. XRD results of clad layers. (a) Fe1, 6 mm/s; (b) Fe2, 6 mm/s;

(c) Fe3, 6 mm/s.

Fig. 7. The delta region of the iron–carbon equilibrium diagram.

4. Discussion


microstructure and the hardness of clad layer

The most interesting thing is that the hardness of

the cladding layer may decrease with the increase of

carbon content when the process parameters are the

same (Fig. 1). For example, when the traverse speed is

6 mm/s, the hardness of the layers decreases from

HRC60 to HRC52 while the carbon content increases

from 0.3 wt.% to 0.4 wt.%. We need to further study

the solidification of the Fe-based alloy in order to

explain the strange phenomenon.

Fig. 7 is the delta region of the iron–carbon equili-

brium diagram [9]. Fig. 7 shows that, with the increase

of carbon content from 0.1% to 0.18% (point H to

point J in Fig. 7), the solidification temperature range

(DT) decreases, DT is the smallest when the carbon

content is 0.18 wt.% and then DT increases with the

increase of carbon content from 0.18 wt.% to

0.51 wt.% (point J to point B in Fig. 7). Obviously,

due to the different grain growth time, the alloy that

has a wider DT will have larger grain size than the

alloy that has a smaller DT. Fig. 3 shows that, under the

same process parameters and with the increase of

carbon content, the average grain size first becomes

small and then becomes big afterwards. From this, the

solidification process of Fe1, Fe2, and Fe3 can be

considered as corresponding to the lines I, II, and III in

Fig. 7. It is worth noting that there is some distance

between the carbon contents in the three kinds of Fe-

based alloy powders and in the lines in Fig. 7. For


example, the carbon content in the peritectic point

(point J in Fig. 7) is 0.18 wt.% while that in the layer

using Fe2 in Fig. 3c, by chemical analysis, is

0.247 wt.%. The small distance between them is

caused by other alloy elements [10].

In the absence of appreciable work hardening, the

relationship between the hardness and the average grain

size of metal material can be described as follows [11]:

Hs ¼ Hi þ Kd�1=2 (1)

where Hs is the hardness of metal material, Hi and K are

the appropriate constants associated with the hardness

measurements.

From Eq. (1) the hardness increases with decreasing

the average grain size. Then the reason that the hardness

of the layers using Fe2 is higher than that of the layers

using Fe3 is that the average grain size of the former is

much smaller than that of the latter, though the carbon

content in the former is less than that in the latter.


microstructure and the cracking susceptibility of

clad layer

The cracks in the laser-clad layers are generally

regarded as hot cracking [4] and Fig. 3f also proves

that the cracks in the Fe-based laser-clad layers are hot

cracking. Previous investigations [12–14] have shown

that, under the same technical conditions, hot cracking

susceptibility is mainly determined by DT and the

plasticity of the alloy. Because of its smallest DT

and high plasticity, due to fine grains, the cladding

alloys using Fe2 have excellent properties of resisting

cracking. Although the cladding alloys using Fe1 have

wider DT than the cladding alloys using Fe2, the

former also have good properties of resisting cracking

because of its high plasticity, due to low carbon con-

tent. Furthermore, as can be seen from the solidifica-

tion of the cladding alloys using Fe2 (line II in Fig. 7),

the peritectic reaction (L0.53 wt.% C + d0.08 wt.% C !g0.18 wt.% C) can exhaust all the liquid phase and d

solid solution at the same time, which will diminish

the harmful effect of the remaining liquid films

between the grain boundaries to the utmost. There

are three reasons that the cladding alloys using Fe3

have high cracking susceptibility. The first is that the

cladding alloys using Fe3 have wide DT. The second is

that it has low plasticity, due to high carbon content.

The last is that, as can be seen from its solidification

(line III in Fig. 7), the grains of the cladding alloy will

contract obviously with the transition of d-Fe to g-Fe

(d + L ! g + L), whereas there is still an amount of

remaining liquid films between grain boundaries at

this time, resulting in a higher cracking susceptibility.

4.3. The best designing principle concerning the

composition and microstructure of Fe-based alloy for

laser cladding

Although many researchers have studied Fe-based

alloys for laser cladding, most of the alloys that they

used can be described as ‘‘high carbon content and

eutectic structure’’ [15–18]. This is the present main

designing principle concerning the composition and

microstructure of Fe-based alloy for laser cladding.

The main characteristics of the designing principle are

as follows:

(1) T
he cladding alloys, with a composition in/or near
the eutectic point, have high carbon content (2.5–

6 wt.%) and then have small DT, fine grains and

relatively high plasticity. As a result, the cladding

alloys exhibit relatively little tendency toward

cracking.

(2) T
he main mechanism of strengthening is fine-
grain strengthening, two-phase strengthening

and solid solution strengthening. The strengthen-

ing phase is alloy carbide, and the matrix is

austenite or ferrite, which is non-equilibrium

phase with extended solid solution alloy elements.

It is known that high carbon content (about

4.3 wt.%) and large quantity of alloy carbides addi-

tions will increase the hardness of the alloy, whereas

they will also impair the plasticity and toughness of

the alloy. Actually, there is seldom example that Fe-

based alloy of ‘‘high carbon content and eutectic

structure’’ has been used in the manufacturing by

laser cladding as yet.

We think the cracks of laser cladding layers have

embodied the deep contradiction between high hard-

ness and good toughness of alloys and the designing

principle of ‘‘low carbon content and peritectic struc-

ture’’ is better than that of ‘‘high carbon content and

eutectic structure’’. The main characteristics of the


designing principle of ‘‘low carbon content and peri-

tectic structure’’ are as follows:

(1) T
he cladding alloys, with a composition in the
peritectic point (point J in Fig. 7), contain low

carbon content (about 0.25 wt.%). The alloys have

small DT and very fine grains.

(2) T
he main structure of the layers is low carbon
martensite with relatively high hardness and good

toughness, and there exits very low carbon ferrite

with high plasticity and toughness between the

grain boundaries, which can absorb and decrease

stresses of the layers, and reduce the cracking

susceptibility.

Without taking measures such as preheating and

tempering, crack-free laser-cladding layers with high

hardness (HRC60) can be obtained by using Fe2.

Recently, we have successfully repaired some huge

rolls of the Pear River Steel Company in Guangdong

province of China. The successful application of Fe2

indicates that our designing principle of ‘‘low carbon

content and peritectic structure’’ is right.

5. Conclusions

Based upon the study, the following conclusions

can be drawn:

(1) T
he hardness of the laser-clad layers does not
always increase with the increase of the carbon

content. Under the same traverse speed, the hard-

ness of layers using the powders containing

0.3 wt.% C is higher than that of layers using

the powders containing 0.2 wt.% C or 0.4 wt.% C.

For all the Fe-based powders used, the hardness of

clad layers increases with increasing the traverse

speed.

(2) T
he cladding layers using the powders containing
not more than 0.3 wt.% C all have good properties

of resisting cracking, while those using the pow-

ders containing more than 0.3 wt.% C have higher

cracking susceptibility. The cracking susceptibil-

ity decreases with a decrease of the traverse speed.

(3) T
he small change of the carbon content in the
alloy powders can change the microstructure and

properties of the layers obviously. The change of

the traverse speed can also change the microstruc-

ture and properties of the layers to some extent.

(4) A
s the carbon content increases from 0.2 wt.% to
0.4 wt.%, the segregation ratio of chromium

increases, while the segregation ratios of nickel,

manganese and silicon first decrease and then

increase. Decreasing grain size will be beneficial

to decreasing solute segregation. At the same

time, with the increase of the carbon content,

the deviations on lattice constant of a-Fe increase.

(5) C
ompared with the designing principle of ‘‘high
carbon content and eutectic structure’’, that of ‘‘low

carbon content and peritectic structure’’ is better.

The laser-clad alloys of ‘‘low carbon content and

peritectic structure’’ have small solidification tem-

perature range, fine grains and small deviation on

lattice constant, and then have high hardness and

good plasticity and toughness simultaneously.

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