This article describes the newest achievements in the heat treatment of diesel-engine fuel-injection nozzles made of hot-working tool steel. Different methods of improving surface properties have been applied by means of vacuum carburizing and vacuum nitriding, which is especially suitable for elements characterized by difficult shape geometry such as blind holes. Variable process parameters have been considered in terms of sequence and temperature as well as their influence on surface microstructure, hardness and case-layer uniformity.
A complex technology was invented involving a thermo-chemical process supplemented by high-pressure gas quenching (HPGQ), deep freezing and tempering. All technological steps were performed in a single-chamber vacuum furnace equipped with low-pressure carburizing (LPC), low-pressure nitriding (LPN) and HPGQ.
Fuel-injection nozzles (Fig. 1) are a key element of a diesel engine that influence performance properties, including fuel consumption and reliability. Furthermore, they play a major role in emission of harmful substances. In the course of cyclic operation, they withstand various loads, work at raised temperatures under high pressures (1,500-3,000 bar) and withstand intense streams of liquids (above 300 feet/second). Due to these factors, nozzles are prone to accelerated wear and defects (Fig. 2). The design of fuel-injection nozzles must ensure appropriate strength, impact, fatigue resistance and abrasion of passage channels.
Nozzles are made of medium- and high-alloy steels featuring surface hardening. Typically, the tensile strength of the core remains within the range of 1,000-1,500 MPa for steel grades 8620, 4320 and 9310, while extended surface hardness (above 60 HRC) is obtained during post-carburization hardening. Adequate heat treatment is performed in vacuum furnaces that feature vacuum carburizing (LPC) and HPGQ (15 bar and above). Vacuum carburizing enables obtaining case uniformity in the thin nozzle channels of complicated shapes, which are inaccessible to conventional carburizing atmospheres. On the other hand, gas quench eliminates the inconvenience of cleaning after quenching in oil. In some solutions, nitriding is applied in place of carburizing to harden the surface.
For certain applications, injection nozzles are made of even more durable steels (e.g., hot-working tool steels). In such cases, the strength increases decidedly (over 2,000 MPa), but the surface still requires additional reinforcement.
The Object and Objective of Tests and Research
The object of testing was injection nozzles made of H11 steel shaped as presented in Fig. 3. The objective of testing was to create a uniformly hardened case layer on the nozzle surfaces (in particular upon the internal surfaces) while maintaining appropriate core hardness. The required case layers were created through vacuum carburizing and nitriding.
A standard SECO/WARWICK Vector Line single-chamber vacuum furnace model 15.0VPT-4035/36IQCN (Fig. 4) was utilized for the tests. The working space was 24 inches high x 36 inches long x 24 inches wide, and it was equipped with LPC and LPN systems and HPGQ (15 bar).
Carburizing was performed with a gas mixture of acetylene (C2H2), ethylene (C2H4) and hydrogen (H2), while ammonia (NH3) was used for nitriding. The 450-pound workload consisted of ballast rods, among which the tested injection nozzles were placed. The workload reflected typical conditions found in industrial heat treatment.
Four heat-treating sequences based on vacuum carburizing were conducted. Carburizing was preceded with prenitriding using PreNitLPC® technology in order to restrict the growth of austenite grains at high temperature. The entire process sequence consisted of: prenitriding, vacuum carburizing (using FineCarb® technology) at various temperatures, which was followed by (direct) quenching in 5-bar nitrogen, deep freezing at -100°F for 2 hours and tempering for 2 hours at 390°F (200°C).
Carburizing treatments were performed at four temperatures: 1580°F (860°C), 1688°F (920°C), 1742°F (950°C) and 1868°F (1020°C), in each case targeted at 0.60% surface carbon and a predefined case depth of approximately 0.016 inch (0.4 mm).
Total times in the sequence of carburizing and diffusion for individual treatments were as follows:
LPC 860 (1580°F) C = 10 minutes D = 70 minutes
LPC 920 (1688°F) C = 4 minutes D = 40 minutes
LPC 950 (1742°F) C = 4 minutes D = 20 minutes
LPC 1020 (1868°F) C = 2 minutes D = 6 minutes
Figure 5 presents a longitudinal cross section through a nozzle following LPC at the temperature of 1688°F (920°C). A uniform case was obtained both on the outer and inner channels of the nozzle, which is a characteristic of vacuum carburizing. The uniformity of the case layers obtained at individual treatments is presented in Fig. 6 as hardness profiles at selected M1-5 points.
After the LPC 860 treatment, a uniformly hardened case layer was obtained on the outer and inner surfaces of the nozzle. The surface hardness was approximately 850 HV at 0.002 inch (0.05 mm), and the core hardness was 500 HV at the predefined case-layer depth of 0.014 inch (0.35 mm) for “core hardness + 50 HV.”
The LPC 920 treatment yielded a uniform case layer of surface hardness 820 HV and case-layer depth of 0.012 inch (0.30 mm) for core hardness of 590 HV.
Also, carburizing at 1742?F (LPC 950) resulted in appropriate uniformity and case-layer parameters: depth 0.014 inch (0.35 mm), surface harness 850 HV and core hardness 620 HV.
It was only the LPC 1020 (1868?F) process that failed to provide satisfactory outcomes. The case layer appeared only on the outer surfaces of the nozzle while none appeared inside. Maximum hardness was 850 HV at the surface and 680 HV at the core. The hardness profile in the outer layer indicates a drop at the surface, which is suggestive of improper microstructure.
Figure 7 presents a comparison of the case-layer microstructure after the individual treatments. A characteristic feature is globular carbides in the matrix of martensite. The carbide size and number increases parallel to the increase of process temperature. They tend to create distinct structures at higher temperatures – a network at austenite grain boundaries – as in the LPC 1020 process at 1868°F.
The nitrided layer was created as a result of a complex process composed of: austenitization at 1888°F (1030°C), quenching in 5-bar nitrogen, tempering at 1076°F (580°C) for 2 hours and vacuum nitriding at the temperature of 1040°F (560°C) for 4 hours.
The results of the process are presented in Fig. 8 as hardness profiles measured at selected spots on the inside and outside surfaces of the nozzle (M1-5). In the course of nitriding, a very uniform hardened case was obtained of surface hardness in excess of 900 HV at the depth of 0.002 inch (0.05 mm) and case depth of nearly 0.007 inch (0.18 mm) at core hardness of 530 HV.
The processes of vacuum carburizing and nitriding enable uniformly hardened case layers to be obtained on H11 tool steel fuel-injection nozzle surfaces with limited access. For vacuum carburizing, the surface hardness exceeds 800 HV for a tempering temperature of 392°F (hardness decreases as tempering temperature rises). The core hardness depends on the quenching temperature and is higher parallel to the rise in temperature. For the tests from 49-59 HRC, it also depends on tempering temperature.
Carburizing at 1868°F (1020°C) failed to create proper uniformity and microstructure (carbide network) due to high intensity of treatment (uncontrollable) and the tendency to create carbides in the steel at higher temperature. LPC processes at that temperature need to be refined.
Promising results were attained for the vacuum nitriding process, including case-layer uniformity and the highest surface hardness (in excess of 900 HV) as well as stability of case-layer parameters at temperatures exceeding 1000°F (538°C).
Based on this study, the case-hardening technology of tool steel fuel-injection nozzles by vacuum carburizing and nitriding has been developed, tested and is ready for industrial implementation.
Further research will focus on improvement of LPC processes at high temperatures and on using hybrid – carburized and nitrided – case layers. IH
For more information: Contact Maciej Korecki, Ph.D. Eng., SECO/WARWICK S.A. Sobieskiego 8, 66-200 Swiebodzin, Poland; tel.: +48 683820506; fax: +48 683820555; e-mail: firstname.lastname@example.org; web: www.secowarwick.com or Professor Piotr Kula, PhD. Eng., Technical University of Lodz, Institute of Materials, Science and Engineering, Stefanowskiego 1/15, 90-924 Lodz, Poland; tel.: +48 426313031; fax: +48 426313038; e-mail: email@example.com; web: http://iim.p.lodz.pl/en/
1. P. J. Blau, N. Yang, “Materials for high pressure fuel injection systems”, US Dept. of Energy, poster presentation May 10, 2011
2. Kula, P.; Pietrasik, R.; Dybowski, K.; Korecki, M.; Olejnik, J. Prenit LPC - the modern technology for automotive, New Challenges. In: Heat Treatment and Surface Engineering, Dubrownik-Cavtat, Croatia, 2009, 165-170.
3. Kula, P.; Korecki, M.; Pietrasik, R.; Wo?owiec, E.; Dybowski, K.; Ko?odziejczyk, ?.; Atraszkiewicz, R.; Krasowski, M. FineCarb® -the flexible system for low pressure carburizing. New options and performance, The Japan Society for Heat Treatment 2009, 49 (1), 133-136.