Effect of salt bath nitriding on microstructure, corrosion resistance, and mechanical

properties of a hot work tool steel

Hangtao Fua, Jin Zhanga,*, Jinfeng Huangb, Yong Liana and Cheng Zhangb

a Institute for Advanced Materials and Technology, University of Science and Technology

Beijing, China

b State Key Lab. for Advanced Metals and Materials, University of Science and Technology

Beijing, China

* Corresponding author at: Institute for Advanced Materials and Technology, University of

Science and Technology Beijing, 100083 Beijing, China

E-mail address: zhangjin@ustb.edu.cn

Tel: +861082377393

Abstract:

In this study a kind of hot work tool steel was modified by salt bath nitriding for 4 h at 540°C

and 560°C respectively and post-oxidation was carried out. Surface and cross-sectional

hardness test results revealed that surface hardness increased apparently after the treatment

because of the formation of compound layer and diffusion zone. Microstructure and phase

analysis showed that more homogeneous compound layer, and more Fe3O4-phase could be

generated after treated at 560°C than at 540°C. As a result, the corrosion potential was

elevated and the self-corrosion current density reduced more obviously. As well, thickness

and porosity of the compound layer also increased with the nitriding temperature elevating.

Due to the solution of nitrogen atom, XRD diffraction peaks broadened and the position of

peaks shifted to lower angle in different degrees at different depths showing the same

mailto:zhangjin@ustb.edu.cn

tendency as hardness curves. Salt bath nitriding deteriorated impact property from 32.3J to

5.2J seriously.

Keywords:

Salt bath nitriding; Hot work tool steel; Microstructure; Diffraction analysis; Corrosion

resistance; Impact property

1. Introduction

Nitriding is one of the most widely used surface treatment modifications which has

established a popular surface engineering method for improving hardness, fatigue strength,

wear and corrosion resistance of various tools [1-4]. Salt bath nitriding gets its popular in

many industrial components [5-7]. In salt bath nitriding, the samples are immersed in cyanate

salt bath, nitrogen atoms diffuse into the substrate through grain boundaries, sub-boundaries

and dislocations driven by chemical potential [8]. Properties are improved due to the

formation compound layer (or so called white layer) and the alloy nitride particles in diffusion

zone. CrN and iron nitrides (Fe2-3N and Fe4N) in the compound layer has outstanding

property of hardness [9, 10], and Fe3O4 formed during nitriding shows favorable properties of

corrosion and wear resistance [6].

The hot work tool steel used in this article is a newly developed low carbon steel

25Cr3Mo2NiSiWVNb. Low content of carbon provides excellent toughness. Chromium and

Nickel could enhance the resistance to heat and combustion. Molybdenum carbide generates

dispersion strengthen effect to enhance temper resistance. Appropriate content of niobium

could refine grains to elevate the toughness and to provide more diffusion paths for nitriding

http://dict.youdao.com/search?q=corrosion&keyfrom=E2Ctranslationhttp://dict.youdao.com/w/resistance/

treatment [11]. Bonding force with nitrogen for elements such as Mo, Cr, W, V, Nb is higher

than that with iron, and the nitrides of these elements shows better wear and combustion

resistance [12]. It is worth of studying the phase and other property changes after salt bath

nitriding on this kind of hot wok tool steel for its further use.

2. Experimental

The material used in this work is a low carbon steel 25Cr3Mo2NiSiWVNb with the

chemical composition shown in Table 1. Before salt bath nitriding, the material was quenched

at 980°C and tempered at 650°C. Samples for tests were machined into dimension of 10 mm

× 10 mm × 55 mm with standard Charpy U-notch for impact test and φ20 mm × 5 mm for

other tests. All surfaces of the samples were grinded by silicon carbide paper until 2000#.

Samples were dipped in the molten salt bath in titanium crucible for 4 h at 540°C and 560°C

respectively for nitriding (CNO- concentration of 35%-37%), and immersed in the oxidizing

salt bath at 400°C for 10 min to eliminate the trace amount of cyanogen. After oxidation, the

samples were cooled down slowly in air to room temperature.

Table 1 Chemical composition of work tool steel 25Cr3Mo2NiSiWVNb (wt.%).

C Cr Mo W Ni Nb V Fe

0.2~0.3 2.7~3.5 1.5~2.4 1.1~1.6 1.0~1.5 0.2~0.3 0.1~0.3 Bal.

Cross-sections of nitrided layers etched by nital (4%HNO3+96%ethanol vol.%) were

observed by ZEISS Imager M2m optical microscope and FEI Quanta 250 Environmental

Scanning Electron Microscope (20kV). Phase structures of surface layer were analyzed by the

type Rigaku X-ray diffractometer (Cu Kα radiation, λ=0.15406 nm, working voltage 40kV,

working current 150mA, step size 0.02°). HXD-1000 Vicker Micro-hardness Tester was used

http://dict.youdao.com/w/concentration/

to measure the surface and cross-sectional hardness. Three indentations were made with 300 g

load for 15 s and the average was calculated to determine each hardness value. The

electrochemical characterization was carried out by potentiodynamic polarization test using a

Versa STAT electrochemical workstation. A standard three electrode cell was used which was

comprised by the sample as the working electrode, a saturated calomel electrode as the

reference electrode and a platinum electrode as the counter electrode. The electrolyte used

was 3.5wt.% NaCl solution. Siemens-TD400C pendulum impact testing machine was used for

the impact test and impact fractures were subjected to the same SEM device mentioned above

to reveal the fracture mechanism.

3. Results and discussion

3.1 Microstructure Analysis

Fig. 1 shows the cross-sectional microstructure of samples treated for 4 h at 540°C and

560°C respectively in different magnification. Nitrided layer consists of compound layer and

diffusion zone as shown in Fig. 1 (a) and (d). For sample treated at 540°C, thickness of

compound layer is about 4 µm with signs of non-uniform thickness as is shown in rectangles

in Fig. 1 (a) and (b). A 7-9 µm thick and continuous compound layer accompanied by a 3-5

µm porous layer in Fig. 1 (f) can be got after treated at 560°C. Under the compound layer, it is

diffusion zone which is about 150-180 µm in thickness. There are some alloy nitrides

developed during the treatment in this zone due to the precipitation of nitrides along with

grain boundaries [12, 13], as shown in circles in Fig. 1 (b) (c) (e) and (f).

Fig. 1 Cross-sectional microstructures of 25Cr3Mo2NiSiWVNb nitriding at different

temperatures: (a), (b), (c) at 540°C and (d), (e), (f) at 560°C.

During salt bath nitriding, the inward diffusion of nitrogen and its subsequent actions

with iron and other alloying elements result in the formation of compound layer consisting of

the corresponding nitrides [14]. Thickness of the compound layer and type of nitrides formed

are largely a function of temperature and the nitrogen chemical potential at surface [15-17]. It

can be seen there is an obvious increase in the depths of compound layer when temperature

was elevated from 540°C to 560°C. This layer is composed mainly of ε-phase (Fe2-3N) [18].

The ε-phase shows perfect corrosion resistance and remains bright under the optical

microscope with etch of mild acid. Elevated temperature of the salt bath resulted in high

concentration of nitrogen on the outside of compound layer. The metastable ε-phase

decomposed and nitrogen atoms were released gradually from the compound layer. As

temperature elevating, nitrogen enrichment on the surface of compound layer enhances the

potential for atoms to escape and exacerbate the porosity. So the degree of porosity is more

apparently at 560°C.These factors led to the porous morphology [6, 19], as shown in Fig. 1 (f).

During the treatment, nitrogen atoms mainly diffuse through grain boundaries and

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sub-boundaries [20, 21]. Relatively faster diffusion speed along the grain boundaries

promotes nitrogen enrichment and nitrides precipitation once the solid solubility limit is

exceeded at this sites. Within the diffusion zone, nitrogen atoms combined with the metal

atoms into nitrides which distribute along with original tempered grain boundaries and

sub-boundaries.

3.2 Diffraction Analysis

Fig. 2 gives the phase composition of the surface layer of samples treated at 540°C and

560°C. It can be observed that this layer consisted mainly of ε-phase (Fe2-3N), together with

some Fe3O4 and CrN in agreement to other researchers [6, 14]. Sample treated at 560°C got

more magnetite Fe3O4 after the treatment than that treated at 540°C which would result in

better corrosion resistance.

Fig. 2 X-ray diffraction pattern of samples treated at 540°C and 560°C.

Fig. 3 shows the X-ray diffraction patterns at different depths below the surface of sample

treated at 560°C. Diffraction peaks broadened and their positions shifted after the treatment.

This can be contributed to an increase in the atomic level lattice strain induced during

treatment [10].Optical microscope shows there are a small number of nitride precipitations

along the boundaries in Fig.1, which is too little to show in the diffraction patterns. Compared

with the untreated sample, all the diffraction peak positions in depths of 30 µm, 60 µm and 90

µm below the surface shifted to lower angle in different degrees. The number of solute

nitrogen atoms decreased as the depth increased. So the position shift decreased successively.

This phenomenon coincides with Bragg equation:

2dsinθ=λ

where d is the lattice plane distance, θ is the diffraction angle, λ is the Cu target’s wavelength.

As wavelength is a constant, the lattice plane distance increases with the diffraction angle (i.e.

θ or 2θ) decreases due to the solute atoms. Number of nitrogen atoms in subsurface is more

than that in deep zone, so position shift according to the depth near the subsurface is more

apparent as shown in Fig. 3. Fig. 4 shows the position shift of the main diffraction peaks (α-Fe

(110)) at different depths for samples treated at 540°C and 560°C compared with untreated. It

can be seen that the two curves intersected at about 70 µm below the surface.

Fig. 3 X-ray diffraction patterns of different depths below the surface of sample treated at

560°C compared with the untreated.

Fig. 4 Position shift of the main diffraction peaks (α-Fe (110)) at different depths below the

surface for samples treated at 540°C and at 560°C.

3.3 Micro-hardness

The changes of micro-hardness were shown in Fig. 5 and Fig. 6. Fig. 5 reveals that both

the treated samples were apparently harder than the untreated one. Surface hardness of

untreated is about 355 HV, which increased to 952 HV and 765 HV after the treatment at

540°C and 560°C respectively. As the formation of porous layer in the treatment of 560°C,

the surface hardness is lower than that treated at lower temperature. After polishing away the

porous layer, the hardness increases to 955 HV. Fig. 6 shows the hardness-depth profile. The

result shows exactly the same tendency as the position shift of the main diffraction peaks

shown in Fig. 4. This phenomenon proves the direct relationships among hardness increase,

peak position shift and lattice distortion caused by the solute nitrogen atoms. It was observed

that the curves can be divided into three parts. Within the depth of 70 µm, sample treated at

540°C shows higher hardness than that treated at 560°C. Beyond this depth, the reducing

trend of the hardness curve along thickness for sample treated at 560°C is smaller than that

treated at 540°C. In the depth of about 240 µm, both the curves increasingly coincide and

then become the same as substrate. Case depth is defined as the distance from the surface

where hardness value is 50 units more than the substrate. It can be seen the case depths are

about 150 µm and 180 µm respectively.

Fig. 5 Surface hardness of samples untreated and treated at 540°C and 560°C.

Fig. 6 Variation of cross-sectional hardness of samples treated for 4 h at 540°C and 560°C.

After the treatment, surface hardness increased significantly because of two factors. One

is the formation of high hardness compound layer, second is the heavy lattice distortion

caused by the diffusion of nitrogen atoms. Continuous decrease of hardness from surface to

the substrate reveals the presences of a diffusion zone, where precipitates of nitrides and other

metallic alloys nitrides are formed at the grain boundaries as well as that in the grains.

Nitrides in grains are too small to be observed by OM or low magnification SEM, which

distort the lattice and pin dislocations and thereby increase the hardness [22]. Formation of

compound layer prevents the contact between salt bath medium and the substrate. And the

precipitates formed along boundaries cut down the major diffusion paths. So the number of

active nitrogen atoms diffused from the medium decreases which could not provide enough

support for further diffusion. Then atoms concentrated in the subsurface (10~70 µm of depth

below the surface) became the main source for further diffusion. Nitrogen atoms disintegrated

from the salt bath medium could not penetrate the compound layer to support the further

diffusion. Diffusion rate for nitrogen atoms increased while the temperature was elevated

from 540°C to 560°C, so more active atoms transformed from the subsurface to deeper

substrate. This illustrates the intersection at the depth of about 70 µm in Fig. 6. More active

nitrogen atoms diffused from the subsurface to deeper substrate at 560°C than at 540°C,

which brings about two results. Firstly, the subsurface lost more nitrogen atoms so the lattice

distortion at 560°C is slighter than at 540°C (within 70 µm), which directly influences the

hardness in this section. Secondly, the deeper zone (beyond 70 µm) could get more active

atoms from the subsurface at 560°C, so hardness curves show the opposite result in this

region. Hardness directly reflects the solid solubility of nitrogen atoms and the degree of

lattice distortion in diffusion zone.

3.4 Corrosion Resistance

Results of potentiodynamic corrosion tests of samples are shown in Fig. 7. Current

density no longer increased with potential when they reach to some extent for treated samples.

This means the passivation tendency occurred in anodic polarization. While the anodic

activation still existed for the untreated sample. The corrosion potential (Ecorr) and corrosion

current density (icorr) are shown in Table 2. The corrosion potential increased about 200 mV at

560°C and the current density decreased about 1/100 compared with the untreated sample.

Thus, corrosion resistance was improved apparently by treated at 560 °C. However, sample

treated at 540°C did not show much improvement in corrosion resistance.

Fig. 7 Potentiodynamic polarization curves of samples untreated and treated at 540°C and

560°C.

Table 2 Ecorr and icorr of samples untreated and treated at 540°C and 560°C.

Ecorr/mV icorr/(A·cm-2)

Untreated -441 3.95×10-6

Treated at 540°C -401 2.21×10-6

Treated at 560°C -252 3.88×10-8

Corrosion resistance improvement is attributed to the formation of magnetite Fe3O4 and

compound layer on the surface. Fe3O4 has good chemical stability and ε-phase could prevent

the substrate from the attack of corrosive medium because of the nobility of nitrides [23, 24].

Although porosity and precipitation of CrN would increase pathways for corrosion[14, 25],

sample treated at 560°C shows an excellent corrosion resistance because the formation of

dense and continuous compound layer is more effective than others and more Fe3O4 was

produced during the treatment. The compound layer of the sample treated at 540°C is

non-uniform as shown in Fig. 1, so the corrosive medium could attack the diffusion zone

directly.

3.5 Impact Property

Energy absorbed by the Charpy U-notch samples in the impact test is shown in Table 3. It

can be seen that the energy absorbed is rapidly reduced for samples treated at both

temperatures compared with the untreated one. The impact fracture morphology of the

untreated and treated at 560°C are shown in Fig. 8. Sample treated at 540°C shows similar

morphology as that treated at 560°C. It can be observed that for the untreated sample, there is

an obvious shear lip on the start of the fracture. The fracture shows morphology of ups and

downs in some degree. But for the one after treatment, the fracture is almost completely flat.

The substrate of both samples shows quasi-cleavage fracture. The most obvious contrast

involves the initiation of the fractures. The obvious ductile fracture with a large number of

small dimples appeared in untreated sample, while the entire cleavage fractures in treated

sample because of the hardened nitriding layer.

Table 3 Impact property of samples untreated and treated at 540°C and 560°C.

Property Untreated Treated at 540°C Treated at 560°C

Energy absorbed/J 32.3±1.2 5.2±0.5 5.2±0.3

The process of fracture occurs in four steps with loading: dislocations movement induces

plastic deformation, crack nucleation, crack growth and fracture. For the untreated sample, a

large number of dislocations migrates which induces obvious plastic deformation to show

shear lip at the onset of the fracture. For the treated sample, dislocations are pinned toughly

by the solute nitrogen atoms, so hardly any plastic deformation can be found and a completely

cleavage fracture was observed [8].

Fig. 8 Impact fracture morphologies of 25Cr3Mo2NiSiWVNb (a) untreated and (b) treated at

560°C.

4. Conclusion

A thin Fe3O4-phase film, compound layer (consist of ε-phase and CrN) and diffusion

zone formed on the surface of 25Cr3Mo2NiSiWVNb after the salt bath nitriding and

post-oxidation, which increased the surface hardness to above 950HV from 355 HV. In the

diffusion zone, solution nitrogen atoms caused diffraction peaks broadened and their positions

shifted to lower angle in different degrees at different depths, which showed the same

tendency as hardness profile curves. Strengthened surface deteriorated impact property

seriously after the salt bath nitriding.

Thickness and porosity of compound layer expanded with the nitriding temperature

increasing from 540°C to 560°C. Sample treated at 560°C got more homogeneous compound

layer and more Fe3O4-phase, and it showed the best corrosion resistance among the untreated

and treated samples.

Acknowledgment

We grateful acknowledge the Chengdu Surface Metal Technology Co. Ltd. for their

facility support of this research work.

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