Microstructural and corrosion characteristics of laser surface-meltedplastics mold steels

C.T. Kwok a, K.I. Leong a, F.T. Cheng b,*, H.C. Man c

a Department of Electromechanical Engineering, Faculty of Science and Technology, University of Macau, Taipa, Macau, Chinab Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

c Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

Received 23 September 2002; received in revised form 16 January 2003

Abstract

Laser surface melting of plastics mold steels P21 (Fe�/3% Ni�/1.5% Mn�/1% Al�/0.3% Si�/0.15% C) and 440C (Fe�/17% Cr�/1.1%

C) was achieved by a 500 W CW Nd:YAG laser using different scanning speeds. The microstructure and the phases present in the

laser surface-melted specimens were analysed by optical microscopy, scanning electron microscopy and X-ray diffractometry,

respectively. The corrosion characteristics of the laser surface-melted specimens in 3.5% NaCl solution and in 1 M sulphuric acid at

23 8C were studied by potentiodynamic polarisation technique. X-ray diffraction spectra showed that laser surface-melted P21 and

440C contain martensite and austenite as the major phase, respectively. Laser surface-melted 440C exhibits passivity whereas laser

surface-melted P21 does not. The corrosion resistance of laser surface-melted P21 in both corrosive media is improved as evidenced

by a lower corrosion current density compared with that of the untreated specimens. The increase in corrosion resistance of laser

surface-melted P21 is due to the dissolution of the intermetallic phase Ni3Al to form a homogeneous solid solution by rapid

solidification. The corrosion resistance of laser surface-melted 440C in NaCl solution is also increased significantly, with the

exhibition of a wide passive range and a low passive current density, but the improvement in sulphuric acid is less pronounced. The

enhanced corrosion resistance of laser surface-melted 440C results from the combined effect of the refinement of carbide particles

with increased C and Cr in solid solution, and the presence of retained austenite. The corrosion characteristics of all the laser

surface-melted specimens are strongly dependent on the laser scanning speed, which in turn results in different microstructures.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Laser surface melting; Nd:YAG laser; Corrosion; Hardness; Plastics mold steels

1. Introduction

Plastics materials constitute an important class of

materials in the consumer product industry. Plastics

mold steels, a new generation of tool steels, are very

important for the plastics processing industry. An

example of the application of plastics mold steels is

the extruder screw for the chemical, medical and food-

stuffs industry. The mold steels require high corrosion

and wear resistances, high hardenability, good polish-

ability and dimensional stability during heat treatment.

The progressive deterioration of plastics mold steels in

acidic and chloride-containing environments leads to

loss of production efficiency and quality, and even

shutdown in serious cases. Corrosion and wear are

common problems in plastics injection molds, especially

in high production molds. Attack arises from acids and

chloride formed from the decomposition of thermoplas-

tics (e.g. PVC) by over-heating [1]. To improve the

corrosion resistance of the mold steels, alloying elements

such as Cr, Ni and Al are introduced. Further improve-

ment in corrosion resistance might be achieved by laser

surface modification via homogenisation and refinement

of microstructure, and/or the formation of new alloys on

the surface.

For more than two decades, research and develop-

ment on the applications of lasers for various surface

modification processes such as laser shock peening, laser

transformation hardening, laser glazing, laser surface

* Corresponding author. Tel.: �/852-27-66-5691; fax: �/852-23-33-

7629.

E-mail address: apaftche@polyu.edu.hk (F.T. Cheng).

Materials Science and Engineering A357 (2003) 94�/103

www.elsevier.com/locate/msea

0921-5093/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0921-5093(03)00228-4

melting (LSM), laser surface alloying and laser surface

cladding have been pursued [2]. Although laser surface

modification processes appear expensive compared with

other surface engineering technologies, in fact they aremore cost effective because of their precision and speed.

LSM is an effective treatment process which is com-

monly used in production lines, for example, in the

automotive industry. In the application of LSM to a cast

iron rocker arm and camshaft, as reported by Bergmann

[3], the laser surface-melted components achieved a

service life four times that achieved by melting using

gas tungsten arc welding. LSM improves the surfaceproperties of materials by rapid melting and subsequent

rapid solidification, resulting in high chemical homo-

geneity and grain refinement. In addition, a localised

area of an engineering component can be modified

selectively, while the properties of the other parts remain

unchanged.

In order to minimise economic losses and to conserve

expensive materials, corrosion control and protection isespecially valuable via prolonging the life of components

undergoing corrosion and wear. LSM has been reported

to be a feasible route for enhancing the hardness and

wear properties of the plastics mold steels [4]. However,

studies related to the effect of LSM on the corrosion

behavior of plastics mold steels are scarce. The efficacy

of LSM of steels on improving corrosion resistance

depends on the system under study [5�/7]. LSM of toolsteels was reported to be capable of increasing the

pitting corrosion resistance in NaCl solutions [5]. LSM

of AISI 420 steel increased the hardness while the

corrosion resistance in NaCl solution was maintained

relative to the starting material (conventionally heat-

treated) [6]. On the other hand, it was reported that

LSM of a Fe�/19% Cr steel degraded its corrosion

resistance in sulphuric acid due to the presence ofinclusions and residual stress [7]. In the present study,

the microstructure and the corrosion behavior of laser

surface-melted plastics mold steels in 3.5% NaCl solu-

tion and 1 M sulphuric acid will be investigated, aiming

at enhancing their corrosion resistance. Two common

mold steels, one containing Cr (440C) and the other

containing Ni and Al but without Cr (P21), will be

investigated. The corrosion resistance of various speci-mens is explained in terms of the microstructural

changes resulting from laser treatment.

2. Experimental details

Two plastics mold steels P21 (UNS T51621) and 440C

(UNS S44004) with different chemical compositions

(Table 1) were selected in the present study. The steelswere in the form of plates with a thickness of 3.4 mm.

P21 was precipitation hardened (320 Hv) whereas 440C

was annealed (260 Hv) in the as-received condition.

Hardened specimens of 440C were prepared by conven-

tional heat treatment in a furnace for comparison with

the laser surface-melted specimens. The as-received

440C was preheated to, and kept at, 850 8C for 45 minand then heat-treated through the austenitising tem-

perature (1060 8C) for 30 min, followed by quenching in

liquid nitrogen. Tempering was achieved by keeping the

specimens at 250 8C for 210 min followed by air-cooling.

The surface of the specimens was sand-blasted before

LSM. The hardened and laser surface-melted specimens

for polarisation studies were cut into square plates of

1.5�/1.5 cm2. Prior to polarisation tests, the surface ofall laser surface-melted specimens was mechanically

polished with 1 mm-diamond paste in order to keep

the surface roughness consistent. Subsequently the

specimens were cleaned, degreased and dried before

the polarisation test.

LSM was carried out using a 500 W CW Nd-YAG

laser with a power of 450 W and a beam size of 1 mm in

diameter (power density: 573 W mm�2). The laser beamwas transmitted by an optical fibre and focused onto the

specimen by a BK-7 glass lens of focal length 80 mm.

The flexible optical fibre delivery was controlled by a

CNC unit. Argon flowing at 20 l min�1 was used as the

shielding gas. Beam scanning speeds (v ) of 5 and 25 mm

s�1 were used. The melted surface was automatically

produced by a CNC program by overlapping successive

melt tracks. The overlapping ratio of the tracks was50%. Such a ratio was chosen as a compromise between

processing efficiency and surface homogeneity.

The laser surface-melted specimens were sectioned,

polished and etched with acidic ferric chloride solution

(25 g FeCl3, 25 ml HCl and 100 ml H2O). The

microstructure of the melted zone was analysed by

optical microscopy and scanning electron microscopy.

In addition, the phases present in the hardened and lasersurface-melted specimens were identified by X-ray

diffractometry (XRD). The radiation source used was

Cu Ka with nickel filter and generated at 1.2 kW.

Microhardness along the depth of the cross-section of

the specimens was determined using a Vickers hardness

tester with a 200 g load and a 15 s loading time.

Cyclic potentiodynamic polarisation scans were car-

ried out using an EG&G PARC 273 corrosion systemaccording to ASTM Standard G61-94 [8] for investigat-

ing the pitting corrosion behaviour. The laser surface-

melted specimens were embedded in a cold-curing epoxy

resin with an exposed area of 1�/1 cm2. Epoxy was also

applied at the interface between the specimen and the

mount in order to prevent crevice corrosion. 3.5% NaCl

solution and 1 M sulphuric acid, kept at a constant

temperature of 23 8C and open to air, were used as thecorrosive media. A saturated calomel electrode (SCE)

was used as the reference electrode and two parallel

graphite rods served as the counter electrode for current

measurement. All data were recorded after an initial

C.T. Kwok et al. / Materials Science and Engineering A357 (2003) 94�/103 95

delay of 30 min for the specimen to reach a steady state.

The potential was increased from 200 mV below thecorrosion potential in the anodic direction at a scan rate

of 5 mV s�1. The scanning direction was then reversed

when an anodic current density of 5 mA cm�2 was

reached and the scan continued until the loop closed at

the protection potential or until the corrosion potential

was reached.

3. Results and discussion

3.1. Microstructural and metallographic analysis

The microstructure of precipitation hardened P21 is

formed of ferrite (a) as shown in Fig. 1(a), with thepresence of a small amount of intermetallic phase Ni3Al

as detected by XRD and shown in the spectrum in Fig.

2(a). For hardened 440C, large primary and finer

secondary carbide particles (M23C6) are observed in

the martensitic matrix as shown in Fig. 1(b). The

presence of these phases was confirmed by the XRD

spectrum in Fig. 2(b). The average hardness of hardened

P21 and 440C is 300 and 790 Hv, respectively.

From the top of the surface, the laser surface-melted

specimens consist of four distinct zones: the melted zone

(MZ) where complete melting occurred, the transition

zone (TZ) where the materials were partially molten, the

heat-affected zone (HAZ) where only solid-state trans-

formation took place, and the substrate. No clear TZ is

observed in laser surface-melted P21. The widths of the

MZ and HAZ of laser surface-melted specimens of P21

and 440C processed at scanning speeds 5 and 25 mm s�1

are shown in Table 2. After LSM, no cracks or pores are

observed in the melt zone (MZ). The cross-sectional

views of the laser surface-melted P21 and 440C at

different scanning speeds are shown in Fig. 3.

For both laser surface-melted steels, owing to the

difference in the quenching rate, completely different

microstructures in the MZ are obtained. The higher is

the scanning speed, the less the total volume melted, the

higher the quenching rate and the finer the dendritic

structure, as is evidenced in the microstructure of

various laser surface-melted specimens (Fig. 4).

For laser surface-melted specimens of P21, lath-type

martensite is present as the major phase, which is

brighter in contrast in Fig. 4(a) (i) and (b) (i). The

intermetallic phase Ni3Al appears as black aggregates in

Fig. 4(b) (i). The presence of martensite as the major

phase and Ni3Al as the minor phase in LM-P21-25 is

supported by the XRD spectrum in Fig. 5(b). For the

specimen processed at a lower scanning speed (LM-P21-

5), the intermetallic phase Ni3Al is absent due to

complete dissolution. For LM-P21-25, owing to a

shorter laser interaction time, some Ni3Al is still left

behind. Dissolution of Ni3Al, whether partial or com-

plete, also increased the tendency of forming martensite.

The microstructure of the laser surface-melted P21 is

finer and more homogeneous than that of the as-

received P21 specimen.

For laser surface-melted specimens of 440C, the

original carbide M23C6 in the substrate dissociated to

form a more homogeneous structure in the MZ and

retained austenite is the major phase while martensite

and precipitated carbides (M23C6 and M7C3) are the

minor phases. Primary austenitic dendrites (bright in

contrast) and interdendritic regions (dark in contrast)

are observed in the laser surface-melted specimens (Fig.

4(c) (i) and (d) (i)). The austenitic dendrites were formed

directly from the liquid phase during rapid solidification

Table 1

Nominal compositions (in wt.%) of plastics mold steels P21 and 440C

Fe Cr Ni Mo Mn Al C Si P S

P21 (UNS T51621) Balance �/ 3.4 �/ 1 1 0.15 1 �/ �/

440C (UNS S44004) Balance 17 �/ 0.75 1.5 �/ 1.1 0.3 0.049 0.03

Fig. 1. Microstructure of (a) hardened P21 and (b) hardened 440C.

C.T. Kwok et al. / Materials Science and Engineering A357 (2003) 94�/10396

[9], with austenite transforming partially into martensite

on rapid cooling. It has been reported that the micro-

structure of laser surface-melted martensitic stainless

steels strongly depends on the laser processing para-

meters [10]. The volume fraction of retained austenite

increases with increasing scanning speed and decreasing

power of the laser beam. The volume fraction of

austenite in LM-440C-25 (v�/25 mm s�1) is higher

than that in LM-440C-5 (v�/5 mm s�1) as indicated by

the SEM micrographs in Fig. 4(c) (ii) and (d) (ii), and

also by the relative intensities of austenite and marten-

site shown in Fig. 5(c) and (d). LM-440C-5 contains

austenite as the major phase, with a higher amount of

martensite but a lower amount of precipitated carbides

compared with LM-440C-25. This finding is consistent

with that of Colaco and Vilar [11]. In Fig. 4(c) (ii), the

SEM micrograph was taken at a location where

martensite was present in a proportion not representa-

tive of the whole sample, in order to show the

morphology of martensite. The major and minor phases,

and the Vickers hardness data at the surface of the

specimens are summarised in Table 3.

A narrow TZ and a wide HAZ are observed below the

MZ in laser surface-melted 440C but only the HAZ isobserved in laser surface-melted P21 (Fig. 6). In the TZ

of laser surface-melted 440C where the maximum

temperature just reached the melting point, localised

melting started and the carbide particles were partially

dissolved, creating large local concentration of carbon in

the austenite [11]. Spheroidal carbide particles are

dispersed in a martensitic matrix (Fig. 6(b)). The HAZ

of laser surface-melted P21 is wider than that of lasersurface-melted 440C because of the higher thermal

conductivity of the P21 (41.3 W m�1 K�1 for P21 vs.

24.2 W m�1 K�1 for 440C).

3.2. Hardness profiles

The hardness profiles along the depth of the MZ of

the laser surface-melted P21 and 440C are shown in Fig.

Fig. 2. XRD spectra of (a) hardened P21 and (b) hardened 440C.

Table 2

Laser parameters and Vickers hardness of hardened and laser surface-melted specimens

Specimens Scanning speed (mm

s�1)

Width of MZ

(mm)

Width of HAZ

(mm)

Average hardness of MZ

(Hv)

Average hardness of HAZ

(Hv)

Hardened P21 �/ �/ �/ �/ 300

LM-P21-5 5 0.61 0.67 350 320

LM-P21-25 25 0.29 0.31 430 420

Hardened

440C

�/ �/ �/ �/ 790

LM-440C-5 5 0.28 0.17 410 500

LM-440C-25 25 0.19 0.17 405 560

C.T. Kwok et al. / Materials Science and Engineering A357 (2003) 94�/103 97

7 and the hardness data are summarised in Table 3. For

laser surface-melted P21, the hardness remains constant

along the melt depth because of the uniformity of

martensite after rapid solidification. The average hard-

ness in the MZ of LM-P21-5 and LM-P21-25 is 350 and

430 Hv, respectively, and is higher than that of the

substrate which was precipitation-hardened (300 Hv). Inaddition, the average hardness of LM-P21-25 is higher

than that of LM-P21-5 because of the presence of

undissolved Ni3Al, and a finer dendritic structure

resulting from the higher scanning speed and quenching

rate.

In the MZ of LM-440C-5 and LM-440C-25, hardness

values of 410 and 405 Hv, respectively, were detected.

Their hardness is higher than that of the substrate (260Hv) but lower than that of 440C hardened by conven-

tional heat treatment (790 Hv) because of the presence

of a high volume fraction of austenite in the laser-

treated specimens. But the hardness of laser surface-

melted 440C is still high because austenite is strength-

ened by solid solution, dislocations and small grains

[11]. The hardest region of the laser surface-melted 440C

is the martensitic HAZ. The hardness of the HAZ ofLM-440C-25 is increased to 560 Hv, which is much

higher than that of MZ (405 Hv).

3.3. Corrosion behaviour

Potentiodynamic polarisation curves of the hardened

and the laser surface-melted specimens in 3.5% NaCl

solution and 1 M H2SO4 (open to air) at 23 8C are

shown in Figs. 8 and 9. Owing to the absence of Cr,uniform corrosion is observed in laser-treated P21 while

pitting corrosion prevails in laser-treated 440C as it

contains Cr. The corrosion potential (Ecorr), the pitting

potential (Epit), and the corrosion or passive current

density (icorr or ipass) extracted from the polarisation

curves are summarised in Table 4.

3.3.1. Laser surface-melted P21

No passivation is observed in hardened and laser

surface-melted P21 specimens in both NaCl solution and

H2SO4 (Fig. 8). In 3.5% NaCl solution, the corrosion

potentials of LM-P21-5 and LM-P21-25 shift in the

noble direction from �/551 to �/467 and �/530 mV,

respectively. Their corrosion resistance is improved as

reflected by a reduction in the corrosion current density

of about six times compared with hardened P21. Basedon the magnitude of the corrosion current density, the

corrosion resistance of the laser surface-melted P21 is

ranked in descending order as follows:

LMP215�LMP2125�Hardened P21

The corrosion resistance of laser surface-melted P21

in 1 M H2SO4 is lower than that in 3.5% NaCl solution.However, the benefit in corrosion resistance in H2SO4

arising from laser treatment is more pronounced, reach-

ing about ten times. In terms of the magnitude of the

Fig. 3. Cross-sectional appearance of laser surface-melted P21 and

440C processed at different scanning speeds (a) LM-P21-5 (v�/5 mm

s�1); (b) LM-P21-25 (v�/25 mm s�1); (c) LM-440C-5 (v�/5 mm s�1);

(d) LM-440C-25 (v�/25 mm s�1).

C.T. Kwok et al. / Materials Science and Engineering A357 (2003) 94�/10398

Fig. 4. Microstructure of the MZ of the laser surface-melted specimens processed at different scanning speeds (a) LM-P21-5: (i) OM, (ii) SEM; (b)

LM-P21-25: (i) OM, (ii) SEM; (c) LM-440C-5: (i) OM, (ii) SEM; (d) LM-440C-25: (i) OM, (ii) SEM.

C.T. Kwok et al. / Materials Science and Engineering A357 (2003) 94�/103 99

corrosion current density, the ranking of corrosion

resistance is the same as in 3.5% NaCl solution.

The improvement in corrosion resistance of the laser

surface-melted specimens in NaCl and H2SO4 obviously

results from the refinement in microstructure and the

homogenisation of chemical compositions by rapid

solidification. As shown in the XRD spectrum in Fig.

5(a), the intermetallic phase Ni3Al was completely

dissociated and dissolved in a homogeneous solid

solution in LM-P21-5 (scanning speed 5 mm s�1),

Fig. 5. XRD spectra for various laser surface-melted specimens.

Table 3

Phases present in hardened and laser surface-melted specimens

Specimens Major phase Minor phases Average hardness at surface (Hv)

Hardened P21 Ferrite Ni3Al 300

LM-P21-5 Martensite �/ 350

LM-P21-25 Martensite Ni3Al 430

Hardened 440C Martensite Austenite, M23C6 790

LM-440C-5 Austenite Martensite, M23C6, M7C3 410

LM-440C-25 Austenite Martensite, M23C6, M7C3 405

C.T. Kwok et al. / Materials Science and Engineering A357 (2003) 94�/103100

resulting in a higher corrosion resistance. On the other

hand, a small amount of Ni3Al is still present in

specimen LM-P21-25 (scanning speed 25 mm s�1).

The intermetallic phase acts as the initiation site for

corrosion attack and its presence deteriorates the

corrosion resistance of LM-P21-25. The corrosion

attack is quite severe in precipitation hardened P21

whereas a much milder damage is observed in the laser

surface-melted specimens in both corrosive media.

3.3.2. Laser surface-melted 440C

Based on the passive current densities, the pitting

corrosion resistance of the laser surface-melted 440C in

both NaCl solution and H2SO4 is ranked in descendingorder as follows:

LM440C25�LM440C5�Hardened 440C

In 3.5% NaCl solution, hardened 440C is active as is

obvious from the polarisation curve. The active behavior

arises from the presence of a large amount of carbideparticles in the martensitic matrix. Since Cr is a carbide-

forming element, the carbide particles are rich in Cr and

the Cr content in solid solution is reduced. As a result

there is not enough Cr to form the passive film on the

hardened specimen. On the other hand, the laser

surface-melted 440C specimens exhibit a wide passive

range and a low corrosion current density. The im-

provement in corrosion resistance is attributed to thedissolution of large carbides, thus increasing C and Cr in

solid solution. In addition, the larger amount of retained

austenite formed by the high scanning speed is an

additional beneficial factor for the increase in pitting

potential [12,13].

In H2SO4, a wide passive range is observed in both

hardened and laser surface-melted 440C specimens,

indicating that passivation is easier for 440C in H2SO4

than in NaCl solution. For all of the specimens, there is

no significant change in the pitting potentials. Com-

pared with the hardened specimen, a smaller passive

current density is observed in the laser surface-melted

specimens (Fig. 9(b)). This could be attributed to higher

Fig. 6. Microstructure of the TZ and HAZ of the laser surface-melted

specimens (a) LM-P21-25; (b) LM-440C-25, OM.

Fig. 7. Hardness profile along the depth of the cross-section of laser surface-melted specimens of (a) P21 and (b) 440C.

C.T. Kwok et al. / Materials Science and Engineering A357 (2003) 94�/103 101

Cr content in solid solution and a larger amount of

retained austenite. The present finding is consistent with

that of Walzak and Sheasby [13] who reported that the

corrosion resistance of heat-treated 440C in dilute

sulphuric acid increases with an increase in austenitising

temperature (i.e. larger amount of retained austenite)

and an increase in the quenching rate. The larger

amount of retained austenite formed, the higher Cr

content in solid solution, and the finer microstructure

due the high quenching rate are the beneficial factors for

the increase in corrosion resistance of laser surface-

melted 440C.

4. Conclusions

Laser treatment of two common mold steels P21 and

440C has been achieved by surface melting and the effect

on the corrosion characteristics in 3.5% NaCl solution

Fig. 8. Potentiodynamic polarisation curves of hardened and laser surface-melted P21 in (a) 3.5% NaCl solution and (b) 1 M H2SO4 at 23 8C, open

to air.

Fig. 9. Potentiodynamic polarisation curves of hardened and laser surface-melted 440C in (a) 3.5% NaCl solution and (b) 1 M H2SO4 at 23 8C.

C.T. Kwok et al. / Materials Science and Engineering A357 (2003) 94�/103102

and 1 M H2SO4 has been studied. The following

conclusions are drawn.

(1) For laser surface-melted P21 processed at a lower

scanning speed (5 mm s�1), only martensite is presentand the intermetallic phase Ni3Al is completely removed

while a small amount of Ni3Al is still present in

specimens processed at a higher scanning speed (25

mm s�1).

(2) The laser surface-melted 440C specimens contain

retained austenite as the major phase, with a small

amount of martensite. On the other hand, the hardened

specimens contain martensite as the major phase.(3) The average hardness in the melt zone of LM-P21-

5 and LM-P21-25 is 350 and 420 Hv, respectively, and is

higher than that of the precipitation-hardened substrate

(300 Hv).

(4) For laser surface-melted 440C, a uniform hardness

of 405 Hv was detected in the melt zone. The hardness is

higher than that of the substrate (260 Hv) but is lower

than that of 440C hardened by conventional heattreatment.

(5) The corrosion characteristics of the laser surface-

melted specimens are strongly dependent on the laser

scanning speed, which in turn results in different

microstructures.

(6) In both 3.5% NaCl solution and 1 M sulphuric

acid, the 440C specimens exhibit passivity whereas the

P21 specimens undergo uniform corrosion. The form ofcorrosion is determined by the composition (i.e. with

and without Cr). Laser treatment in these two mold

steels essentially does not change the form of corrosion,

but changes the rate.

(7) For laser surface-melted P21, the corrosion

resistance in both media is improved as reflected by a

reduction in the corrosion current densities. The im-

provement in corrosion resistance is due to the removalof the intermetallic phase Ni3Al, resulting in a homo-

geneous solid solution.

(8) The enhanced corrosion resistance of laser surface-

melted 440C in both media is reflected by a reduction in

the passive current density arising from the combined

effect of carbide refinement, the increase of C and Cr in

solid solution, and the presence of retained austenite.

Acknowledgements

The authors wish to acknowledge the support fromthe infrastructure of the University of Macau and the

Hong Kong Polytechnic University.

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Table 4

Corrosion parameters of hardened and laser surface-melted specimens in 3.5% NaCl solution and 1 M H2SO4 at 23 8C, open to air

Specimen In 3.5% NaCl In 1 M H2SO4

Ecorr (mV) icorr (mA cm�2) Epit (mV) Ecorr (mV) icorr or ipass (mA cm�2) Epit (mV)

Hardened P21 �/551 30 Active �/450 700 Active

LM-P21-5 �/467 5 Active �/460 70 Active

LM-P21-25 �/530 10 Active �/433 100 Active

Hardened 440C �/280 0.8 Active �/520 80 900

LM-440C-5 �/310 0.6 35 �/509 50 900

LM-440C-25 �/301 0.3 265 �/508 50 900

C.T. Kwok et al. / Materials Science and Engineering A357 (2003) 94�/103 103