Restoration of Working Surfaces for Forming Elements from Molds for High-Pressure Casting of Non-Ferrous Metals by Laser Surfacing ^†

Abstract

The article presents a study of the possibilities for restoring the working surfaces of molds for high-pressure casting of non-ferrous metals by laser surfacing using a filler material. Test specimens with parameters of real forming elements were manufactured. The influence of nitriding on welded layers and basic material was studied in comparison with one without nitriding. The X-ray diffraction method was used to obtain the phase composition of the surface of the samples in areas submitted to nitriding. Scanning electron microscopy (SEM) was used to determine the microstructure of the nitriding layers, welded layers and bulk material. Energy-dispersive X-ray spectrometry (EDX) was applied to investigate the chemical composition in the welded and nitrided zone. Mechanical property means of microhardness measurements were studied. Four zones were identified after nitriding in the weld area. The first zone closest to the surface with a thickness of 0.025 mm is characterized by a higher microhardness, which reaches 700 HV. The second zone, which is part of the diffusion zone, is 0.1 mm thick, and it is characterized by the same size grains with a similar shape and microhardness, which reaches 600 HV. The microhardness measured in the welded zone after nitriding is 50% greater than that without nitriding. The thickness of the diffusion zone in 1.2343 steel reaches about 0.15 mm, and the microhardness is about 900 HV near the edge, but quickly decreases to 400 HV.

1. Introduction

During their operation, molds for high-pressure casting of non-ferrous metals are subjected to intense mechanical, thermal and chemical stress. The melt enters them at high speeds of 130–140 m/s at pressures reaching 130–140 MPa. In combination with the high temperature of the molten alloy above 650 °C, when casting aluminum alloys, the surface layer of the forming elements is heated to over 500 °C, which leads to erosion, abrasion and corrosion of the mold. During the casting cycle, the mold is also subjected to cyclic thermal stress, leading to the appearance of tensile stresses in the surface layer of the working elements, which leads to the development of thermal fatigue of the material, respectively, to the appearance of cracks in the forming surfaces [1,2]. The solid inclusions dissolved in the molten metal, such as Al₂O₃, in combination with the high velocity of the melt and, in most cases, the turbulent nature of the flow entering the working cavities, lead to abrasive cavitation wear of the surface layers of the working elements [3,4,5] (see Figure 1).

In most cases, there is a combination of unfavorable operating conditions, and the tools go out of service before they have reached the specified number of working cycles. Premature wear of the forming elements leads to serious economic losses caused by the high cost of the molds, the missing profits from the production of parts and the need for large quantities of spare parts. To reduce these losses and prevent frequent emergency repairs of the tooling equipment, work is being performed in several directions:

-

optimization of the tool designs in relation to the casting system [6,7,8];

-

use of tool materials corresponding to the high intensification of the processes with appropriate modification of the surface layers [9,10,11,12];

-

appropriate thermal and chemical–thermal treatments [10,11,12,13];

-

development of technologies for the repair of already worn-out forming elements [14,15].

One of the technological solutions for restoring worn areas of forming elements is laser surfacing of the damaged area with filler material. This technology can be used to repair complex and expensive parts with local damage, most often caused by erosive processes [16,17].

Bonek et al. [18,19] studied the modification of the microstructure of hot-work tool steel X40CrMoV5-1 during the surface modification by means of laser technology. They remelted the surface of the steel and alloyed it with tungsten carbides to reinforce the structure. Lee et al. [20] improved the surface hardness and wear properties of AISI H13 tool steel through solid hardening and refinement of microstructure using laser source. Grum and Slabe [21] made a tool-repair comparison between laser surfacing and arc surfacing. There are no literature sources for studies of combined technologies based on laser surfacing and subsequent nitriding of heat-resistant steels.

The report presents a study of laser surfaced layers using filler metal on test specimens made of heat-resistant steel grade 1.2343 from BOHLER, heat-treated and nitriding. It focuses on the study of the laser surfaced section of the specimens.

2. Materials and Methods

2.1. Material and Experimental Samples

For the needs of the study, cylindrical specimens of heat-resistant tool steel grade 1.2343 from the company BOHLER were prepared. The test specimens with shape and dimensions are presented in Figure 2. A wedge section was cut along the periphery of the test specimens by wire erosion with a “V” section, imitating eroded material from the base, which was filled with filler material by laser welding.

The chemical composition of the base material is presented in

Table 1. Chemical composition of steel 1.2343.

C	Si	Mn	Cr	Mo	P	S
0.39	0.95	0.31	4.66	1.14	0.017	0.004

. It was studied using a BELEG LAB 3000s spectrometer (Belec Spektrometrie Opto-Elektronik GmbH, Georgsmarienhütte, Germany).

The filler material is a product of the company JOTKE (GmbH, Georgsmarienhütte, Germany), with catalog number jotke Fill W-22, suitable for laser welding of steel 1.2343, with a hardness of the base material HRc 38-58.

2.2. Technological Process for Manufacturing Experimental Specimens

2.2.1. Mechanical Processing

Turning of bar material with grade 1.2343 to obtain blanks serving as the basis for the test specimens.

2.2.2. Heat Treatment

The heat treatment was carried out in a HTS brand vacuum furnace. Heat treatment was carried out in the furnace including quenching and triple tempering mode. The parameters of the mode were designed and simulated using software [22]. The graphs from the simulations are presented in Figure 3 and Figure 4. Based on the simulations, a control program of the facility was created, presented in Figure 5.

2.2.3. Thermo-Chemical Treatment

The samples were subjected to subsequent thermo-chemical treatment—nitriding. It was carried out in a combined furnace of the “CODERE” brand, presented in Figure 5, with the following parameters:

-

Nitriding temperature—580 °C;

-

Duration of the process in an ammonia environment—3 h;

-

Ratio of the working gases in the chamber ammonia/nitrogen—30/70.

The diagram of the nitriding process carried out in the “CODERE” furnace is presented in Figure 6.

2.2.4. Cutting a Segment Along the Periphery of the Test Body

Using EDM, a segment is cut from the surface with the shape and dimensions imitating an eroded area.

2.2.5. Laser Surfacing

Is performed on a JOKE laser installation, mod. ENESKAlasser 1000.

2.2.6. Secondary Nitriding of the Test Specimens

Secondary nitriding of the test bodies was carried out for the purpose of thermo-chemical treatment of the welded area.

2.3. Phase Analysis

To perform a phase analysis of the resulting laser welded and nitrided layers, an X-ray diffractometer, Bruker D8 Advance (Bruker, Billerica, MA, USA), was used. The used method was “TwoTheta” Bragg–Brentano. The characteristics of X-ray radiation was CrKα, with a wavelength of 2.291 Å. A step of 0.05$°$ and a registration time of 1 s per step was performed. All the measurements were applied on the surface of the samples in two areas: (1) in the welded and nitrided zone; (2) in the nitrided zone. The X-ray beam had a collimator with a diameter of 1 mm.

2.4. Scanning Electron Microscopy (SEM)

The structure of the laser welded and nitrided samples were investigated by using a scanning electron microscope (SEM) Zeiss Evo 10 (Oberkochen, Germany). For microstructural analysis a secondary electron detector was used, the parameters used for the best metallographic pictures are as follows: WD = 8.5 mm and EHT = 10 kV. For energy-dispersive analysis a back-scattered detector was used with higher EHT, because of the need to be activated more electrons form the sample to achieve a better signal for the different chemical elements.

2.5. Macrohardness

A ZHVµ Zwick/Roell microhardness tester (Ulm, Germany) was used to determine microhardness values in cross section direction in depth using loading force 0.05 kgf for a holding time 10 s. The microhardness was measured in cross section, with three measurements in a line for each zone that has been investigated.

3. Results and Discussion

Gas nitriding was carried out on steel 1.2343. The steel is stamped and heat-resistant. The main structure of the steel is pearlitic. The presence of chromium helps to develop a deep diffusion zone when conducting gas nitriding. The main technological parameters in nitriding are the nitriding temperature, saturation time and degree of dissociation (respectively, partial pressure) of the nitrogen-containing gas. The alloying elements also have a significant influence on the saturation process. The main influence is exerted by aluminum, vanadium, molybdenum and chromium. In pearlitic steels, it is the alloying elements forming nitrides that have the main influence on the nitrided layer. Steels with an increased content of alloying elements of this type contribute to the formation of a strongly pronounced wear-resistant diffusion zone, without obtaining a white layer on the surface of γ′ nitride and ε nitride. The thickness of the diffusion zone in steel 1.2343 reaches about 0.15 mm (Figure 7a), and the hardness reaches about 900 HV. The thickness of the layer during carbonitriding can reach up to 0.25 mm. At the beginning of the process, the diffusion zone develops rapidly but does not reach the required hardness. As the process continues, the thickness of the diffusion zone increases slightly, but the mechanical properties change (the hardness increases).

In this case, the saturation time is relatively short to obtain maximum hardness (it is also necessary to follow the degree of gas dissociation in the furnace). Figure 7a shows the diffusion zone after nitriding of the base material. There are no distinct nitride zones on the structure. The strengthened zone is a mechanical mixture of alloyed nitrides ε-(Fe, Cr_2-3N) and γ′ (Fe, Cr₄N). A small amount of ferrite is recorded on the diffractogram (Figure 8).

A channel is cut into a cylindrical steel 1.2343, which is welded with an electrode with a high nickel content (

Table 2. Chemical composition obtained from EDX analysis after laser surfacing and nitriding.

Fe	Ni	Co	Mo	Ti	Cu
56.2	17.2	10.7	4.3	1.7	1.5

and Figure 9). The cylindrical sample is pre-nitrided; after welding on the area of the cylinder, an additional nitriding is carried out. The welded layer builds an austenite zone in the welding area, due to the presence of components that expand and stabilize the austenite zone, nickel, nitrogen and carbon.

The action of nickel is to stabilize austenite, which as a phase with a crystal lattice dissolves a significant amount of nitrogen in the octahedral cavities. Nitrogen additionally stabilizes the austenite phase, leading to saturation of the crystal lattice upon cooling and a phase hardening occurs. The distortion of the crystal lattice is observed in the changed intensities of the X-ray lines of the diffractogram. The presence of molybdenum and iron in the zone forms dispersed nitrides in the diffusion zone, but due to their dispersion they cannot be detected by the diffractometer (Figure 10).

In Figure 7b, four zones are clearly distinguished. The first zone closest to the surface with a thickness of 0.025 mm is characterized by a higher microhardness, which reaches 700 HV. The increased microhardness may be due to the greater saturation of the surface layers with nitrogen. The resulting structure in the diffusion zone also differs in the grain size. The structure is finer near the surface, which leads to an increase in microhardness. The second zone, which is part of the diffusion zone, is 0.1 mm thick, and it is characterized by the same size grains with a similar shape. The measured microhardness in this zone is a constant value of 600 HV. The third zone is about 1 mm thick, and it is from the deposited filler material.

From the EDX analysis, it is noticeable that the amount of Cr in this zone is increased, while Ni and Co are in a smaller amount compared to the first two zones. The measured microhardness in this zone is about 580 HV. The fourth zone, which is visible in Figure 7b, is formed from steel 1.2343. The microhardness smoothly decreases from 580 HV to 420 HV within 2 mm of the surface, after which it remains constant.

Figure 11a shows the measured microhardness in a sample subjected only to welding with filler material, while Figure 11b shows the measured microhardness after laser welding and subsequent nitriding. The microhardness was measured in the radial direction from the surface of the samples to the core. The measurement was performed in three zones: (1) zone filled with filler material; (2) zone with welded material on the surface of the sample and (3) zone without filler material.

Based on the results obtained, the following conclusions can be drawn:

-

the maximum obtained microhardness is in the zone without filler material, subjected to nitriding;

-

in the sample subjected to laser welding, the maximum microhardness is in the zone between the added material and the base metal;

-

in the sample subjected to final nitriding, the microhardness reaches its maximum in the surface layers in all three zones;

-

the measured microhardness in the welded zone after final nitriding increases by 50% compared to where nitriding was not performed.

4. Conclusions

Gas nitriding was carried out on steel 1.2343. The presence of chromium helps to develop a deep diffusion zone when conducting gas nitriding. The alloying elements also have a significant influence on the saturation process. Steels with an increased content of alloying elements of this type contribute to the formation of a strongly pronounced wear-resistant diffusion zone, in this case without obtaining a white layer on the surface of γ′ nitride and ε nitride. In the diffusion zone after the nitriding of the base material, there are no clearly defined nitride zones on the structure. The strengthened zone is a mechanical mixture of alloyed nitrides ε-(Fe, Cr_2-3N) and γ′ (Fe, Cr₄N). A small amount of ferrite is recorded on the diffractogram. The welded layer builds an austenite zone in the welding area, due to the presence of components that expand and stabilize the austenite zone, nickel, nitrogen and carbon. The action of nickel is to stabilize austenite, which as a phase with a crystal lattice dissolves a significant amount of nitrogen in the octahedral cavities. Nitrogen additionally stabilizes the austenite phase, leading to saturation of the crystal lattice upon cooling and a phase hardening occurs. The distortion of the crystal lattice is observed in the changed intensities of the X-ray lines of the diffractogram. The presence of molybdenum and iron in the zone forms dispersed nitrides in the diffusion zone, but due to their dispersion they cannot be detected by the diffractometer.

There is a difference in the microhardness obtained and the depth of the diffusion zones after nitriding. Four zones were identified after nitriding in the weld area. The first zone closest to the surface with a thickness of 0.025 mm is characterized by a higher microhardness, which reaches 700 HV. The second zone, which is part of the diffusion zone, is 0.1 mm thick and it is characterized by the same size grains with a similar shape and microhardness, which reaches 600 HV. The thickness of the diffusion zone in 1.2343 steel reaches about 0.15 mm, and the microhardness is about 900 HV near the edge, but quickly decreases to 400 HV.