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LOW-TEMPERATURE NITRIDING OF PRECIPITATION HARDENED CORROSION RESISTANT TOOL STEELS

F. Calosso1, C. Ernst2, U. Huchel3

1Politecnico di Torino, Sede Alessandria, Alessandria, Italy

2Edelstahl Witten-Krefeld GmbH, R & D, Witten, Germany 3Eltro GmbH, Heat Treatment, Baesweiler, Germany

Abstract For some years modifications of the conventional plasma nitriding process have been used to nitride austenitic and ferritic corrosion resistant steels. By the thermochemical surface treatment at low temperatures, their wear resistance has been improved without causing a deterioration in corrosion behaviour. Up to now, just minor experience with the application of this process to precipitation hardened martensitic tool steels has been gained. The paper presents a study on the pulsed plasma nitriding, nitrocarburizing and carburizing treatment of plastic mould steel Thyroplast PH X SUPRA with special regard to employing low temperatures. A description of the pulsed plasma process is given considering the parameters to create the optimized conditions within the ELTRO-CORR process for this precipitation hardened martensitic steel. 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 depths 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.

Keywords Plasma nitriding, plastic mould steel, microstructure, corrosion resistance, wear

INTRODUCTION Nitrocarburizing or nitriding via liquid baths or the gas phase are widely used surface

hardening technique in the tool steel sector. Depending on the steel to be treated, the thermochemical process takes place at temperatures of 450-550°C (gas nitriding) or 570-590°C

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(salt bath nitrocarburizing). As during the treatment chromium containing precipitations are formed that facilitate corrosive attacks during tool application, such surface treatments are generally not recommended to be applied to corrosion resistant tool steels. Also, the surface treatment in conventional gases has not become very popular for these types of steels as their passive layer is chemically resistant and therefore no nitriding effect takes place [1]. However, there is a selection of new nitriding processes available which have been developed during the past twenty years like the plasma nitriding process. It can be operated at low temperatures and by surface sputtering also allows the treatment of corrosion resistant steels.

Figure 1 : Section of a pulsed plasma nitriding furnace and schematic process diagram [2]

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 hundreds 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 [2]. 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 a typical furnace for plasma surface treatments and a schematic process diagram.

EXPERIMENTAL For the plasma treatments, the corrosion resistant plastic mould steel Thyroplast PH X

SUPRA was used (Table 1). 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

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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).

Table 1 : Chemical composition of steel Thyroplast PH X SUPRA

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.

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.

Chemical composition in mass.-% Material

C Si Mn P Cr Mo Ni Cu Nb Thyroplast PH X SUPRA 0,022 0,25 0,45 0,014 14,7 0,1 5,1 2,5 0,16

Parameters Speci-men

Plasma surface treatment Time [h] Temperature [°C]

U6 carburized 12 440 W2 nitrided 12 440 W3 nitrided 12 410 V2 nitrocarburized 12 440 V3 nitrocarburized 16 440 V4 nitrocarburized 20 440 V1 nitrocarburized 16 500

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RESULTS By GDOS analysis typical profiles of carbon and nitrogen as well as of the other elements

were determinded for all of the seven different surface treatments. As an example, Figure 2 presents the concentration in dependence of the depth for specimen V2 that has been nitrocarburized 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.

Figure 2 : 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.

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

Sample Treatment Depth [µm] Surface hardness [HV0,1]

U0 None 0 390 U6 carburized 25 - 30 950 W2 nitrided 24 – 28 1080 W3 nitrided 12 – 16 1160 V2 nitrocarburized 45 – 50 1180 V3 nitrocarburized 50 – 55 1090 V4 nitrocarburized 60 – 65 1100 V1 nitrocarburized 55 – 60 1210

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With most of the plasma treatment parameters applied, i.e. nitriding and nitrocarburizing 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 nitrocarburized specimen V2 (440°C / 12 h) is given in Figure 3. The microstructure contained two diffusion zones of the elements nitrogen (~ 20 µm) and carbon (~ 45 µm), their different penetration depths clearly becoming visible.

Figure 3 : Microstructure of specimen V2 (nitrocarburized 440°C/12 h), optical microscopy

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 4).

Figure 4 : Microstructure of specimen V1 (nitrocarburized 500°C / 16 h), optical microscopy

Nitrogen diffusion zone

Carbon diffusion zone

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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 5, 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 carburized specimen (U6, 440°C / 12 h). In contrast, nitrocarburizing and nitriding 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.

Figure 5 : Pitting potential of Thyroplast PH X SUPRA in chloride containing solution

Finally, the wear behaviour of untreated and plasma nitrocarburized 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 6 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

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surface, the surfaces of the nitrocarburized 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.

0,00,20,40,60,81,01,21,41,61,82,0

1000 10000 100000 1000000Number of revolutions

Wea

r lo

ss [

g]

no treatment

440°C / 12h

440°C / 16h

440°C / 20h

Figure 6 : Adhesive wear in the untreated and nitrocarburized condition, “Amsler-Test”

CONCLUSIONS For some years now modifications of the conventional plasma nitriding process have been

used to surface treat austenitic and ferritic corrosion resistant steels at low temperatures, while just minor experience has been gained with the treatment of martensitic corrosion resistant steels [3, 4].

By applying the ELTRO-CORR process, the precipitation hardened martensitic tool steel Thyroplast PH X SUPRA was successfully surface treated. 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 steel investigated 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.

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REFERENCES [1] Christiansen, R., Somers, M.: Randschichthärtung von rostfreiem Stahl durch

Gasnitrierung und Gascarburierung bei niedrigen Temperaturen. Z. Werkst. Wärmebeh. Fertigung 60 (2005) 4, pp. 207 – 214.

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

[3] 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.

[4] 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.