**3. Effects of molybdenum on hardening properties of stainless steels by low temperature vacuum carburizing and pre-treatment**

Recent CO2 regulations for light-duty passenger cars, especially in Europe, decreases from 130 g/km (2015), 95 (2021), 80 (2025) to 59.4 (2030) [24].

#### *Current Development of Automotive Powertrain Components for Low Friction and Wear… DOI: http://dx.doi.org/10.5772/intechopen.106032*

Automakers endeavor to improve powertrain design for fuel efficiency and satisfy emissions requirements. The fuel injector is the main component of precise fuel metering to control combustion characteristics and reduce emissions. Currently, fuel injectors are developed with higher pressures to meet the CO2 regulation and get competitiveness. The injector is composed of the coil and the needle assembly, which has a stopper, armature, position ring, needle bar, and ball. Materials of injector shall have resistances for high pressure, high temperature, corrosion, and abrasion. The stopper and position ring are continuously impacted by the vertical motion of adjacent parts during operation and are currently applying expensive carburizing heat treatment [21, 25, 26]. Thus, it is necessary to develop a cost-effective heat treatment. New developed carburizing has a marginally different hardening principle from conventional carburizing methods. The structure of current carburized austenitic stainless steel is that carbon is dissolved in the metal matrix to increase the surface hardness caused by high compressive stresses, whereas there is no carbide formation to worsen the corrosion resistance [27] and weldability. Concerning expanded austenite structure, that is, S-phase, nitrogen atoms diffused into the face-centered cubic (fcc) lattice at low temperature, and the S-phase containing layer has high carrion resistance and high surface hardness [28, 29]. The objectives of this work were to develop low-temperature vacuum carburizing and their acid etching of stainless steels for modern injector parts.

As experimental and results, the concerned parts were stopper (SUS303, 0.05 C-0.3 Si-1.9 Mn-0.03 P-0.32 S-17.2 Cr-8.5 Ni-0.25 wt% Mo) and position ring (1.4305, 0.05 C-0.3 Si-1.9 Mn-0.03 P-0.31 S-17.6 Cr-8.6 Ni-0.4 Cu-0.4 wt% Mo), which made by stainless steel containing 2 wt% Mn for machinability. At first, currently applied parts were measured, carburized layer thickness of 21.3–24.1 μm and hardness HV0.05 914–959.

To substitute the current low-temperature gas carburizing process, new lowtemperature vacuum carburizing, and acid etching pre-treatment were developed to reduce the cost and improve product quality.

Stainless steels with more than 12 wt% chromium have Cr2O3 passivation layer with corrosion resistance, but this layer plays as a barrier layer for carburizing. The pre-treatments for deletion of passive layer, for example, acid etching, NH4Cl, plasma, and halogen gas, are necessary. Acid etching was chosen and it was to find their optimal condition, for example, acid media, concentration, and duration. Moreover, the objective was to avoid the formed soot during carburizing. Soot causes failure at laser welding with the formation of pores and the reduction of corrosion resistance [30]. In this work, low-temperature vacuum carburizing was performed in a commercial vacuum carburizing furnace (VH556–10, Rübig, Austria). High purity acetylene (C2H2, 99.90%) and hydrogen (H2, 99.999%) were used as the process gases. Low-temperature vacuum carburizing was carried out for 24 h (including heating and cooling times) with a carburizing potential (Kc) of 0.32 at a working pressure of 800 Pa and a temperature of 450°C. Process conditions were partly used as reported in previous work [25, 26], and further optimized.

Especially, SUS303 and 1.4305 showed different pitting and oxide regeneration behavior by acid etching regarding the chemical composition difference: Stopper (SUS303) and position ring (1.4305) were tested with the variation of acid concentration and duration. As a result, the position ring (1.4305) had lower pitting than that of the stopper (SUS303) and showed carburizing behavior. From diverse acid concentrations (high/middle/low) in nitric-hydrofluoric acid, the mid concentration

#### **Figure 7.**

*Carburized stopper (SUS303) as cross section (50x, left) and surface layer (500x, right) (mid acid etching of pH 1.67).*

of pH 1.67 and short time of 55 seconds had no pitting and enabled carburizing on all surfaces with the hardness of HV0.05 902–942, **Figure 7**.

In the case of stopper SUS303, when the acid etching time was short, the carburized layer was not formed. Oppositely, if the time was increased beyond a certain level, the carburized layer was formed, but its hardness had a low value between HV0.05 500 and 600, the reason was that MnS inclusions at the surface led to severe MnS pitting over time. However, in 1.4305 material, at the same time as the abovementioned acid etching was performed, not only the pitting was disabled, but also the carburized layer was completely formed on the surface.

As a result of application to the product, the position ring (1.4305) had lower pitting and homogenous carburized thickness and hardness than those of the stopper (SUS303) under the same acid condition, **Figure 8**. For a detailed declaration of this reason, the number, average size, and composition fraction of MnS inclusions were analyzed by optical microscope (OM) and scanning electron microscope-energy-dispersive X-ray spectroscopy (SEM-EDX). The surface before and after acid etching was analyzed by X-ray photoelectron spectroscopy (XPS). OM Image analyzer resulted from position ring (1.4305) had more and bigger MnS inclusions in position ring than those of stopper. EDX mapping confirmed that Mn, S distribution at stopper and position ring had no significant differences. However, the difference in chemical composition, especially, molybdenum, was shown.

One of the reasons for different pitting behavior was the different Mo content and related pitting resistance equivalent number (PREN) (position ring: 1.4305, 0.4 wt% Mo, stopper: SUS303, and 0.25 wt%). To investigate other reasons, the Mo and Mo-oxide contents of both steels before and after acid etching were analyzed by XPS in-depth, **Table 1** and **Figure 9**. There was no difference before etching, but after etching there was a significant changed in Mo and Mo-oxide composition in depth. And Mo of the position ring (1.4305) was higher at the surface and in-depth as well, **Figure 9**. In detail, the composition of formed oxide layers at the position ring and stopper were different. Position ring had MoO2, MoO3 at the surface, and Mo, MoO2, and MoO3 in-depth were higher than those of the stopper. Thus, high Mo content led to high pitting resistance, and the formation of a fast Mo-oxide layer because of higher gibbs free energy than Cr2O3 [31].

The relatively high content of molybdenum in the 1.4305 steel formed Mo-oxides on the surface during acid etching, which the excessive pitting by the MnS inclusion site was prevented. Furthermore, these oxides (mainly MoO3) resolved easily by hydrogen during low-temperature vacuum carburizing and subsequently enabled activated carburizing [32].

*Current Development of Automotive Powertrain Components for Low Friction and Wear… DOI: http://dx.doi.org/10.5772/intechopen.106032*

#### **Figure 8.**

*Difference of pitting of carburized layer under same acid condition (left: Stopper (SUS303), right: Position ring (1.4305)).*


#### **Table 1.**

*XPS results from surface to depth of stopper (SUS303) and position ring (1.4305).*

In conclusion, 1.4305 showed outstanding carburizing properties, hardness of HV0.05 911–1059, the thickness of 20–25 μm, including satisfaction of weldability and low roughness of Rz 0.809 μm, **Figure 10**. In final, stopper material was changed to position ring material 1.4305, and subsequently the test for mass production is carried out.

In summary, the objectives of this work were to develop low-temperature vacuum carburizing and their acid etching for injector parts, which were stopper (SUS303) and position ring (1.4305). Currently applied parts had the carburized thickness of 21.3–24.1 μm and the hardness of HV0.05 914–959. The new carburizing and acid etching should enhance the hardness-related wear resistance and durability, the reduction of production cost, and product quality. As an experimental result, SUS303 and 1.4305 showed different pitting and oxide regeneration behavior by acid etching due to the material composition difference. In SUS303, when the acid etching time was short, the carburized layer was not formed. If the time was increased, the carburized layer was formed, but its hardness was low, the reason is that MnS inclusions at the surface led to severe MnS pitting. In 1.4305, at the same time as the acid etching pre-treatment was performed, not only the pitting was suppressed, but the carburized layer was completely formed on the surface. The relatively high content of molybdenum in 1.4305 formed Mo-oxides on the surface during acid etching, which the excessive pitting by the MnS inclusion site was prevented. Furthermore, these oxides (mainly MoO3) were resolved

**Figure 9.**

*XPS comparison of each element (Cr, Mo, O) in weight percent (stopper vs. position ring). a) Stopper initial. b) Stopper after acid etching. c) Position ring initial. d) Position ring after acid etching.*

**Figure 10.** *Developed pre-treated and carburized layer on position ring (1.4305).*

*Current Development of Automotive Powertrain Components for Low Friction and Wear… DOI: http://dx.doi.org/10.5772/intechopen.106032*

easily by hydrogen during carburizing and subsequently enabled activated carburizing [32]. In conclusion, 1.4305 showed excellent carburizing properties: hardness of HV0.05 911–1059, thickness of 20–25 μm, satisfaction of weldability, and low roughness. Therefore, the stopper material was changed to position ring material 1.4305.

#### **4. Conclusions**

From our previous studies on various coating systems [6, 7, 14, 15, 17, 19, 20], the mass production of a nanocomposite coating could be easily possible by using a single alloying target. Furthermore, it is considered that the nanocomposite coating could be prepared with the different phases that possess the desired properties by designing the composition of the alloying target and controlling the conditions of the coating process. Now, it has been tried to develop new coating systems suitable for harsh environments, such as non-lubrication conditions and heavy-loading conditions. Also, the mechanism for the catalytic effects of Cu has not been revealed at all and the effective amount and structure of Cu in the coating should have been studied. It will be discussed in other manuscripts. As advanced surface lubrication, the nanocomposite coatings can substitute the current applied coatings, for example, DLC, SiO-DLC, and ta-C, in near future. With the consideration of economic aspects and reality, coating and heat treatment technology for automotive powertrain components have been developed for low friction and wear reduction. Concerning coating technology injector balls are SiO-DLC coated with PVD and PACVD and proper jig for low friction and wear. For the achievement of coating quality, pre-treatment of coating, and cleaning is essential. Therefore, the improvement of cleaning for the balls was done: the most effective cleaning was the pre-treatment of acetone cleaning in the ultrasonic bath, DI water boiling, and acetone cleaning in ultrasonic bath before the current cleaning process. In particular, the fluorescence analyzers clearly clarified the cleanliness level. Concerning heat treatment technology, low-temperature vacuum carburizing and pre-treatment for injector parts were developed and showed during acid etching Mo-oxides on the surface are formed, especially by 1.4305 with high molybdenum content. Through these Mo-oxides with easy resolution behavior, carburizing was promoted. The changed stopper steel (1.4305) was appropriate for the new vacuum carburizing and their acid etching.

#### **Acknowledgements**

This study has been conducted with the support of the Korea Institute of Industrial Technology as "Development of intelligent root technology with add-on modules" (KITECH EO-22-0005).

The work "Importance on pre-treatment of coating for using the smallest spherical parts of powertrain fuel systems" was carried out with the collaboration of coating company DONGWOO HST.

The part "Effects of molybdenum on hardening properties of stainless steels by lowtemperature vacuum carburizing and pre-treatment" was conducted with institutions

and companies (KITECH, Dongwoo HST, Samlak). Especially, Dr. Jun-Ho Kim from KITECH is truly appreciated for his collaboration.
