Surface modification of steels used in valve manufacturing

Introduction

In many industrial sectors, the quality of pipes and especially shut-off and control valves (SCV) is critical not only for the uninterrupted transportation of liquids and gases but also for the overall accident-free operation of production facilities [1–4]. Protective coatings on the surface of metal and alloy products, obtained through ion chemical-thermal treatment (ICTT) methods, significantly improve their performance characteristics. One of the most promising variants of ICTT is ion nitriding [5–9]. The literature refers to this process by several names: ion-vacuum, ion-plasma, and ion nitriding [10–13]. This universal method of surface modification in a glow discharge plasma in a vacuum [14–16] is relatively simple to apply, allows for the strengthening of all types of steels and alloys, and is considered a “white metallurgy” process [17].

Engineers from the Israeli company “HABONIM” were among the first to apply low-temperature ion-plasma nitriding (IPN) to enhance the wear resistance of valve components [18]. Today, the technology of low-temperature plasma carbonitriding for ball valves is specified in all catalogs of the Israeli manufacturer [19].

Currently, the Austrians are recognized leaders in the field of ion nitriding. They have developed and patented technologies such as PLASNIT, PLASOX, and PLAPOL, which are also used to strengthen valve components [20].

Although research on the ICTT process has been conducted in Russia since the 1970s, the active use of ion nitriding in industry began relatively recently [21–24]. The production of equipment in Russia also started much later than in other countries, but today there are domestic IPN units available. Automated software based on technological know-how, the use of microprocessor and high-precision equipment, as well as technologies for high-speed transmission of large amounts of information and modular robotic systems, allows them to compete with foreign counterparts [21].

As of 2020, the share of imports in the consumption structure of industrial SCV in Russia was 53 % [3]. In 2023, it is still necessary to import the missing product positions [25], so domestic enterprises are independently mastering new types of products and seeking new technologies to improve the reliability and quality of valve products. In this regard, comparing results obtained on units from different manufacturers, as well as conducting research and summarizing data on the strengthening of steels by ICTT, are currently pressing tasks.

The aim of this work is to study the modified layers obtained by ion-plasma nitriding on steels used in valve construction and to compare the data obtained on Bulgarian and Russian production units.

Research methodology

The ion-plasma nitriding (IPN) process was conducted using equipment from IONITECH (Bulgaria) and Ion Technologies LLC (Russia). The IPN temperatures ranged from 550 to 580 °C, the pressure was 4 mbar, and the gas mixture consisted of 25 % N₂ and 75 % H₂. The isothermal holding time (5–12 h) was determined based on the four different steel grades selected for the study. The holding duration was the same for identical samples on both setups. Fig. 1 shows images of parts during the nitriding process.

The study was conducted on steels of grades AISI 431, AISI 420, AISI 301 and AISI 321, which are used for manufacturing SCV parts such as gates, seats, stems, separators, pistons, spindles, wedges, etc. The general characteristics of the steels are provided in Table 1. For destructive testing, witness samples were placed in the working chamber along with the parts.

After IPN, the entire surface of the samples was visually inspected for external defects. The nitrided surface was checked for color uniformity, and the absence of peeling and chipping, especially along sharp edges, at a magnification of 15–30 times. The brittleness of the nitrided layer was controlled by examining the diamond pyramid indentation according to the VIAM brittleness scale (STo INTI S.70.2-2022).

One of the key indicators of strengthening is surface hardness. This value represents the hardness of the thin modified layer, so it is important to select a load that ensures the indenter does not penetrate through the surface layer (STo INTI S.70.2-2022). Preparation for measuring this indicator involved smoothing the surface with P2500 grinding paper, after which the surface hardness of the layer was determined using the Vickers method according to GOST 2999-75 under a load of 49.03 N. Surface microhardness [26] was assessed under a load of 0.98 N according to GOST 9450-76 using a PMT-3 device (Russia).

Samples for microstructural analysis were embedded in Bakelite powder and prepared into slides according to the standard methodology (STo INTI S.70.2-2022). Microstructural examination was carried out using a BiOptic microscope (Russia) with a universal etchant (4 % nitric acid) to reveal the nitrided layer. According to standards, the total depth of the layer (h_h) is determined by measuring microhardness from the surface in a direction perpendicular to it until the microhardness matches the core microhardness value. The layer depth was controlled by the durometric method on a PMT-3 microhardness tester under a load of 0.98 N with a hold time of 10 s (STo INTI S.70.2-2022).

Results and discussion

The results of the study on nitrided layers on steels of different compositions (see Table 1), obtained using ion-plasma equipment from Bulgarian and domestic manufacturers, are presented in Fig. 2.

Ion-plasma nitriding (IPN) resulted in a uniformly developed, non-brittle nitrided layer on the parts. Visual inspection showed that the parts and samples had a uniform matte gray color without any surface defects. Studies of steel sections with more than 12 % Cr content revealed a modified zone that is easily etched, with clear boundaries. The depth of the nitrided layers, as determined by microstructure, reaches h_s = 0.08÷0.22 mm (Fig. 3), while the depth determined by microhardness distribution is h_h = 0.10÷0.25 mm. In all cases, the depth h_h of the nitrided layer obtained on the domestic equipment is greater.

For steels with martensitic and martensitic-ferritic structures (see Table 1), there are minimal differences in the depth of the nitrided layer between the microstructure and microhardness for layers obtained in both cases (Fig. 3), although the results using domestic equipment are higher. For austenitic-martensitic and austenitic steels (see Table 1), the differences in the depth of the nitrided layers are significant (Fig. 3), indicating that part of the nitrided layer (the transition zone) is not revealed by etching with the reagents used. Based on the obtained data, it can be concluded that the method of determining the depth of the nitrided layer by microhardness is more informative and accurate.

Table 2 presents the results of measuring the surface hardness and microhardness of the nitrided layers, which are the main criteria for the wear resistance of shut-off and control valves.

Modifying the metal surface led to an increase in the surface hardness of all the investigated steels. During the process of diffusion saturation of the surface with nitrogen, the formation of iron nitrides and alloying elements occurred, resulting in the increased hardness of the nitrided layer. The maximum result was observed in AISI 301 steel: its surface microhardness increased fivefold from ~260 HV_0.1 to ~1200 HV_0.1. The minimum increase in microhardness (2.8 times) was noted in AISI 321 steel.

Thus, the surface hardening technology allowed for the preservation of the dimensional and surface finish accuracy of the parts [27]. Three of the largest Russian SCV manufacturers [3] are already using ion nitriding to improve the quality of their products and are incorporating it into their production cycles [28].

Conclusions

The commonly used steels in valve construction, AISI 420, AISI 301, AISI 431 and AISI 321, show an increase in surface hardness by 2.8 to 5 times after ion-plasma nitriding under the conditions of the conducted studies.

The method of assessing the depth of the nitrided layer by microhardness is more accurate than determining it by microstructure. This is because, during the nitriding of austenitic-martensitic and austenitic steels (see Table 1, Fig. 3), part of this layer (the transition zone) is not revealed by etching.

A comparative analysis of the microhardness of the modified layers established that the Russian equipment produced by Ion Technologies LLC performs on par with foreign equipment under identical operating conditions. It demonstrates comparable surface hardening results for parts to those achieved with well-known global market equipment from IONITECH (Bulgaria).