Low-temperature Plasma nitriding

Uwe Huchel 1 ,Claudia Ernst 2 , Siegfried Strämke 1
1 Eltro GmbH, Heat Treatment, Arnold-Sommerfeld-Ring 3, D – 52499 Baesweiler, Germany
2 Edelstahl Witten-Krefeld GmbH, R & D, Auestrasse 4,
D – 58452 Witten, Germany

Abstract

Austenitic stainless steels have an excellent corrosion resistance, but the wear resistance is very poor. Standard nitriding processes obtain a higher hardness but lower the corrosion resistance. The ELTRO-CORR process improve both, the wear and the corrosion resistance. The paper mainly presents a study of the Eltro GmbH with the Edelstahl Witten-Krefeld GmbH on the Eltro-CORR treatment of precipitation hardened plastic mould steel Thyroplast PH X SUPRA. The success of the low temperature treatment has been verified by measurements of microhardness, glow discharge surface spectrometry (GDOS) as well as by analysis of the microstructure using optical microscopy and scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX). Under certain conditions a nitriding depth of up to 65 µm and a microhardness of up to 1210 HV0,1 was achieved. Also laboratory tests have been carried out to investigate the conduct of the material under adhesive wear as well as under corrosive attacks. Precipitation free specimens did not show any decrease in pitting corrosion behaviour while wear resistance was significantly improved.

1.Introduction

The use of plasma technologies offers important advanta-ges to the user. With Pulsed Plasma Nitriding and the Pulsed Plasma Nitrocarburizing processes well defined layers for wear resistance and for corrosion resistance can be reproducibly produced. The operating lifetime and heat resistance of the part are improved. Austenitic stainless steels have an excellent corrosion resistance, but the wear resistance is very poor. Standard nitriding processes obtain a higher hardness but lower the corrosion resistance. Low temperature plasma nitriding can improve both, corrosion and wear resistance. The metallurgical background is based on expanded austenite (S-phase). The S-phase can be created by carbon and by nitrogen. The nitrogen S-phase provides higher hardness and lower toughness. In the case of a carbon S-phase the hardness is lower and the toughness is higher. [1,2,3] The variation of carbon and nitrogen offers a wide range of process possibilities, metallurgical results and properties.
In plasma nitriding at pressures of around 50 to 500 Pa taking place in a nitrogen-hydrogen atmosphere, the plasma is generated in the vacuum chamber between the parts (cathode) and the receptacle (anode) at several hundred volts. Today practically all industrial furnaces used for plasma nitriding are equipped with the pulsed discharge technology. Pulsing lowers the energy input into the furnace and the temperature uniformity of the load is improved. Typical values for the pulse duration are 50 to 100 µsec and for the pulse repetition time about 100 to 300 µsec [4]. Depending on the mixture of process gases, i.e. H2 and N2, the composition of the formed layers can be controlled. By adding small amounts of carbon containing gases such as CH4 and CO2, also plasma nitrocarburizing processes may be carried out. Figure 1 shows schematic a typical furnace for plasma surface treatments, Figure 2 schematic process diagram.

Plasma Unit
Plasma Unit

Fig. 1 Schematic furnace diagramm

Process diagram
Process diagram

Figure 2: Schematic process diagram

2. Experimental

Table1 shows typical hardness values for an austenitic stainless steel (1.4571) after the Eltro-Corr treatment. The hardness increased without loss of corrosion resistance.

Suface hardness [HV0,1]

untreated                     180
Eltro – Corr N             935
Eltro – Corr N + C     1100
Eltro – Corr C             500
Table 1: Typical hardness values for an austenitic stainless steel (1.4571) after the Eltro-Corr treatment at 430°C.
For further investigations, the corrosion resistant plastic mould steel Thyroplast PH X SUPRA was used. This steel had a very low carbon content of up to 0,04 mass.-%, a chromium content of 15 mass.-% and a copper addition of 2,5 mass.-% in order to be suitable for precipitation hardening. All specimens were taken from an electroslag-remelted, forged block of dimension 650 x 350 mm. A typical solution annealing at 1040°C/2h/air and aging at 520°C/4h was carried out to reach a hardness of 40 HRC (375 HB).
In this condition the steel presents a very high corrosion resistance exceeding that of conventional martensitic stainless tool steels such as X38CrMo16 (material-no. 1.2316), hence it is used in the field of processing of “corrosive” plastics such as PVC, most notably in plastic injection moulding and extrusion. Due to the increasing use of filler and strengthening materials contained in plastics nowadays, the demands not just on corrosion resistance but also on wear resistance are high. As by normal heat treatment the hardness of Thyroplast PH X SUPRA cannot be increased to values higher than 44 HRC, some plasma surface treatments applying the ELTRO-CORR process were carried out aiming at increasing the hardness and wear resistance while retaining the good corrosion resistance of the material. Table 2 summarises the parameters of the different treatments.

table2

Table 2: Overview on process parameters

To characterize the treated surfaces, glow discharge optical spectroscopy (GDOS) analysis to determine concentration profiles of the elements, Vickers hardness measurements to identify the depth and the hardness of the diffusion layers as well as optical microscopy were applied. Corrosion resistance was investigated by determination of the pitting potential in different chloride containing aqueous solutions. Adhesive wear was tested in the “Amsler-Test” applying a force of 500 N and a velocity of the rolls of 175 to 194 rpm. As contact partner for the surface treated specimens, high speed specimens made of steel HS10-4-3-10 (material-no. 1.3207) with a hardness of 67 HRC were used. Finally, additional investigations by scanning electron microscope (SEM) combined with energy dispersive X-ray analysis (EDX) were carried out in order to analyse the chemical composition of the matrix and the precipitations.

3. Results

By GDOS analysis typical profiles of carbon and nitrogen as well as of the other elements were determined for all of the seven different surface treatments. As an example, Figure 3 presents the concentration in dependence of the depth for specimen V2 that has been treated at 440°C for 12 h. While nitrogen showed a constantly decreasing concentration from the surface in direction of the bulk material, an enrichment of carbon was found at some distance from the surface.

fig3

Figure 3: GDOS analysis of specimen V2 (nitrocarburized 440°C/12 h)

The total depth of the diffusion layer, which was 45 µm in this case, was confirmed by Vickers hardness measurements of the cross-section in HV0,1. A summary of the measured depths as well as of the hardness values at the surface is given in Table 3.

table3

Table 3: Layer depths and surface hardness of differently treated specimens

With most of the plasma treatment parameters applied, at temperatures between 410°C and 440°C for up to 20 h, a precipitation free diffusion layer without compound layer could be produced. A representative overview on the specimen V2 (440°C / 12 h) is given in Figure 4.

fig4

Figure 4: Microstructure of specimen V2 (Eltro-Corr N+C 440°C/12 h), optical microscopy

The microstructure contained two diffusion zones of the elements nitrogen (~ 20 µm) and carbon (~ 45 µm), their different penetration depths clearly becoming visible.
The only case where some severe precipitations were detected in the diffusion zone was the nitrocarburized specimen V1 that had been treated at 500°C for 16 h. Here, a certain amount of precipitations was already observed during the inspection of cross-sections by optical microscopy (Figure 5).

fig5

Figure 5: Microstructure of specimen V1 (Eltro-Corr N+C 500°C / 16 h), optical microscopy
A detailed analysis by SEM and EDX confirmed that these phases were chromium rich (44,8 mass.-%), nitrogen containing particles with a size of up to 4 µm and a globular to spicular morphology.

In some of the specimens also cracks parallel to the surfaces were detected. They were attributed to some preparative problems as well as to the hardness gradient between the treated surface region and the bulk material. Particularly, the cracks appeared in the region where hardness drops down significantly, this will be subject to further optimization.
As in the processing of plastics, the surfaces of a tool are predestinated for getting into contact with some corrosive media, for example chloride ions, electrochemical measurements were conducted to compare the behaviour of Thyroplast PH X SUPRA in the untreated and surface treated condition. In Figure 6, the current density in a deaerated aqueous solution containing 1000 ppm Cl-, measured at room temperature and with a feed of dU/dt = 200 mV/h, is shown in dependence of the applied potential. It becomes clear that the pitting potential of the untreated specimen (+130 mV) was moved to a less noble potential (+40 mV) in case of a precipitation containing specimen (V1, 500°C / 16 h) or to even a negative potential (-100 mV) in case of a Eltro-Corr C specimen (U6, 440°C / 12 h). In contrast, Eltro-Corr N and N+C plasma treatments at temperatures of 410°C to 440°C with times of up to 20 h which resulted in compound layer and precipitation free surfaces did not cause any major change in the pitting corrosion behaviour.

fig6

Figure 6: Pitting potential of Thyroplast PH X SUPRA in chloride containing solution
Finally the wear behaviour of untreated and plasma treated specimens was compared in the “Amsler-Test”. In this test adhesion between the two metal surfaces and mechanical load results in surface spalling that in practice finally causes the breakdown of tools. Figure 7 shows the wear loss of the specimens in dependence of the number of revolution. While the untreated specimen showed plastic deformation and spalling on the surface, the surfaces of the Eltro-Corr N+C specimens were much less deteriorated and the material loss due to adhesive wear was significantly smaller. These results seem to be in accordance with the different hardness of the tested specimens.

fig7

Figure 7: Adhesive wear in the untreated and nitrocarburized condition, “Amsler-Test”

4. Conclusions

By applying the ELTRO-CORR process the hardening in the surface region could be accurately controlled by treatment time, temperature and gas composition in order to avoid precipitation of chromium rich nitrides or carbides. Starting from a hardness of the untreated material of 390 HV0,1, surface values between 950 HV0,1 and 1210 HV0,1 were reached depending on the composition of the layer, i.e. contents of nitrogen and carbon. The Thyroplast PH X SUPRA showed a potential for producing usefully thick layers with a depth of up to 65 µm. They offer the option of significantly increasing the wear resistance of tools applied in plastic injection moulding and extrusion while maintaining the excellent corrosion resistance of Thyroplast PH X SUPRA.
Subsequently, hardness gradients may be tailored by an appropriate combination of nitrogen and carbon contents in the surface layer to allow a smooth transition from the hard surface zone to the soft bulk material.

References

[1] Spies, H.J., Eckstein, C et al: Korrosion und Randgefüge nichtrostender Stähle nach einer Tieftemperaturnitrierung. HTM 57 (2002) 6, pp. 268-273

[2] Leyland, A. et al: Low temperature plasma diffusion treatment of stainless steels for improved wear resistance. Surface and Coatings Technology, 62 (1993), pp. 608 – 617.

[3] Kuwahara, H. et al: Plasma nitriding of Fe-18Cr-9Ni in the range of 723 – 823 K. Oxidation of Metals, 36 (1991),
pp.143-156.

[4] Liedtke, D., Huchel, U. et al: Wärmebehandlung von Eisenwerkstoffen, Nitrieren und Nitrocarburieren. 3rd edition, Expertverlag (2005).