The processing difficulties of connecting shaft parts are analyzed, and the processing methods are improved. The focus is on the precautions for machining the nitrided surface. By blunting the edges of the parts before nitriding, the stress situation during nitriding is improved, which effectively solves the problem that the edges of the parts are easy to break off when machining after nitriding, and ensures the processing quality of the parts.
PART 1
Introduction
Nitriding is the process of infiltrating nitrogen atoms into the surface layer of a workpiece, which changes the chemical composition of the surface layer and forms a layer mainly composed of nitrides, thereby improving the surface hardness, wear resistance, fatigue resistance and anti-galling properties of the parts[1]. After gas nitriding, the surface hardness of the parts can reach 1000 HV (about 70 HRC) under a 50 N load, and it can still maintain high hardness and high wear resistance at a temperature of 600℃. After nitriding, the surface of the parts can obtain greater residual compressive stress, and the fatigue strength can also be greatly improved[2], which is unmatched by other chemical heat treatments. Nitriding is categorized into gas nitriding, liquid nitriding, and ion nitriding, among others. Our company currently primarily uses gas nitriding. Compared to liquid nitriding, gas nitriding offers easier atmosphere control (using ammonia as the nitriding medium), produces fewer harmful gases that threaten human health and the environment, and exhibits more stable quality, thus making it more widely used. Commonly used materials for nitrided parts include 18Cr2Ni4WA, 38CrMoAlA, 40CrNiMoA, 25Cr3MoA, S106, S132, 1Cr11Ni2W2MoV, 0Cr17Ni4Cu4Nb (17-4PH), 00Ni18Co8Mo5TiAl, TM210A, 0Cr15Ni5Cu4Nb (15-5PH), and 0Cr13Ni8Mo2Al (PH13-8Mo), and are typically used in applications requiring wear resistance, such as joints, supports, and rocker arms.
Nitriding is divided into overall nitriding and localized nitriding. Localized nitriding should be avoided as much as possible during design. If localized nitriding is necessary, one of the following methods can be used for anti-nitriding treatment: ① Allow for machining allowance of more than twice the nitriding depth. ② Plate a tin layer with a thickness of 0.003–0.015 mm. ③ Plate a non-porous copper layer with a thickness of more than 0.02 mm. ④ Plate a nickel layer with a thickness of 0.02–0.04 mm. ⑤ Apply an anti-nitriding coating.
The surface roughness of the parts before nitriding should meet the requirements of the drawing. Generally, the surface roughness value Ra should be controlled between 0.4 and 0.8 μm. The surface should be clean and free of oil stains, rust spots, and dents, especially sharp edges. The machining allowance of the workpiece should meet the process specifications. Generally, the grinding allowance on one side of the nitrided surface of structural parts should be ≤0.05 mm.
The nitriding process temperature is relatively low, generally 460–650℃. Nitriding atmosphere control is very important. Abnormal atmosphere can lead to defects such as insufficient nitriding layer depth, nitriding layer network structure, and loose compound [3]. Since nitriding does not undergo quenching or other treatments that cause large deformation, the deformation is small, which is a major advantage of the nitriding process and is very beneficial for parts that will not be processed after nitriding. The main reason for the dimensional instability of nitriding is the change in structure and residual stress, as well as the micro-plastic deformation that occurs under service conditions. Generally, the deformation of the nitrided surface is positively correlated with the thickness of the nitrided layer, which is about 1/10 of the thickness of the nitrided layer of the part. When compiling the process flow, the influence of deformation on the dimensions should be fully recognized. Dimensions with small tolerances must be precision machined after nitriding.
PART 2
Analysis of the difficulties in machining connecting shaft parts
The connecting shaft shown in Figure 1 is used in the flow feedback device of the servo variable hydraulic motor such as weapon hatch and flap. It is the connecting bridge between the swashplate assembly and the sensor. Its function is to accurately transmit the rotation angle torque of the swashplate assembly to the sensor and monitor the flow of the hydraulic motor. The part is made of 0Cr17Ni4Cu4Nb precipitation-hardening stainless steel. The outer circular surfaces of the right and left ends (φmm and φmm respectively) are required to be nitrided to a depth of 0.1–0.3 mm. The hardness of the nitrided surface should be ≥58 HRC, while the hardness of the non-nitrided surface and the core should be 31–39 HRC. The transition points between the two sealing grooves and one retaining ring groove on the outer circular surface require polishing with a radius of R0.1–R0.2 mm. Due to the high hardness and brittleness of the nitrided surface, the machining difficulty lies in the machining of the nitrided surface and the treatment of the transition fillets between the nitrided and non-nitrided surfaces. Inappropriate process flow and machining methods can lead to quality problems such as chipping at sharp edges. Therefore, the rationality of the process plan is crucial.
Figure 1 Connecting Shaft
PART 3
Selection of Finishing Methods
Nitrided surfaces possess high hardness, high strength, high brittleness, wear resistance, and corrosion resistance, and have good chemical stability, but their machinability is relatively poor. Nitrided surfaces are typical hard and brittle materials. Improper machining methods can damage the surface layer structure of the workpiece, making high-quality machining of nitrided surfaces a technical challenge.
The main machining methods for nitrided surfaces are turning, milling, and grinding. Considering tool wear resistance and tool life, PCBN (polycrystalline cubic boron nitride) inserts should be used for turning and milling whenever possible. Polycrystalline cubic boron nitride inserts have a hardness of 3500–4500 HV and a heat resistance temperature of 1250–1350℃. They exhibit exceptional chemical inertness, good toughness and thermal conductivity, a low coefficient of friction, and strong anti-adhesion properties, making them particularly suitable for machining hardened steel, cobalt-based materials, and nickel-based materials that are difficult to cut [4, 5]. In tool path design, the tool should enter from the outside of the workpiece towards the solid part, avoiding the path from the solid part towards the outside. Even so, edge chipping cannot be completely eliminated.
Grinding should be the preferred machining method when the part structure allows. When selecting grinding wheels, those with excessively high hardness should generally be avoided. Excessive hardness leads to a rapid temperature rise at the contact point, causing thermal stress on the workpiece surface at high grinding temperatures, ultimately resulting in residual tensile stress and a high risk of grinding cracks. White fused alumina grinding wheels offer good cutting performance, but their lower toughness allows for easy abrasive grain shedding. For nitrided surface grinding, white fused alumina wheels are superior to single-crystal fused alumina wheels [6-8].
External cylindrical grinding methods are divided into longitudinal grinding and transverse grinding. While longitudinal grinding has lower efficiency, it produces better surface quality and a lower surface roughness. Transverse grinding, while more efficient, requires greater grinding force and higher temperatures, necessitating a sufficient supply of cutting fluid. For nitrided surface grinding, longitudinal grinding is preferred, despite its lower efficiency, as heat is more easily dissipated during the grinding process, reducing the likelihood of grinding cracks.
PART 4
Process Flow
When compiling the process specifications for nitrided stainless steel parts, the division of processing stages and the selection of anti-nitriding methods should be fully considered. Simultaneously, stress-relief processes should be arranged in appropriate locations to eliminate part deformation caused by processing stress. Grinding should be used as much as possible for nitrided surface processing. Grinding generates compressive stress on the workpiece surface, while turning and milling generate tensile stress. Grinding is more likely to ensure the integrity of the part surface. The process flow can generally be divided into the following stages:
(1) Gear blank forming: The blank is a forging or bar stock.
(2) Rough machining: Removing a significant amount of excess material.
(3) Solution treatment and precipitation hardening: Ensuring the hardness requirements of the non-nitrided surface.
(4) Semi-finishing: Removing the black scale from the heat-treated surface, leaving a small allowance for finishing.
(5) Stress Relief Treatment: For complex, thin-walled, precision, and large-diameter parts, stress relief treatment should be performed after roughing or semi-finishing to reduce deformation during nitriding (multiple treatments may be necessary). A certain machining allowance should be left before stress relief.
(6) Copper Plating: Copper plating is applied for protection, with an overall copper plating thickness of 30–50 μm.
(7) Semi-finishing: The copper layer on the nitrided surface is removed, completing the surface machining for nitriding.
(8) Gas Nitriding: Surface nitriding is completed. For particularly precise or easily deformable parts, a certain grinding allowance is left before nitriding, and the copper is removed by grinding after nitriding.
(9) Copper Removal: All copper layers on the surface of the part are removed.
(10) Finishing: The nitrided surface and precision dimensions are finished.
PART 5
Machining Flow and Existing Problems Before Process Optimization
The main machining flow of the connecting shaft before process optimization was: CNC turning of the φ10mm outer diameter and groove → rough milling of the φ5mm outer diameter and part outline → copper plating → CNC turning to remove the copper layer from the surface of the φ10mm outer diameter → vertical machining center to remove the copper layer from the surface of the φ5mm outer diameter → nitriding → copper stripping → external cylindrical grinding of the φ10mm outer diameter → polishing of the fillet.
Because the fillet radius R0.15mm at the junction of the φ10mm outer diameter and the 3.4mm wide groove, and the radius R0.5mm at the sharp edge of the φ5mm outer diameter junction, after machining on the external cylindrical grinding machine and coordinate grinding machine, R0.15mm and R0.5mm are no longer complete. Although sharp edges are produced after grinding, polishing is required to meet the fillet requirements. However, the fillet tolerance is small, and it is easy to exceed the tolerance during polishing. Furthermore, due to the excessive removal of allowance during fillet polishing, chipping is very likely to occur at the edges. Meanwhile, the annular groove on the outer circle will undergo slight deformation after nitriding. If the annular groove is machined to the final size required by the drawing before nitriding, the opening of the annular groove will shrink after nitriding, resulting in dimensional deviations.
PART 6
Optimized Process Scheme
(1) Optimization method for φ10mm outer circle: To ensure the integrity of the rounded corners after grinding, the rounded corners at the transition between the φ10mm outer circle and the sealing groove need to be geometrically processed during rough machining. A conical transition is added to the rounded corner, with the depth of the conical surface according to the grinding allowance and the angle of the conical surface according to 10°～20°. The conical surface and the side of the sealing groove are transitioned with rounded corners (the final rounded corners required by the drawing). This ensures that after removing the grinding allowance during finishing, the transition rounded corners are completely preserved, and the edge chips will not be caused by excessive removal of allowance at the rounded corners during subsequent polishing. Actual verification shows that after adding the protective transition conical surface, the chipping phenomenon is completely eliminated, ensuring the machining quality of the parts. Figure 2 shows the CNC turning of the outer circle and sealing groove before process improvement. Figure 3 shows the geometric treatment of the sealing groove's sharp edge after process improvement.
Figure 2: CNC turning of the outer circle and sealing groove before process improvement
Figure 3: Geometric treatment of the sealing groove's sharp edge after process improvement
(2) Optimization method for φ5mm outer circle: The same treatment method is used for the φ5mm outer circle. A 12° conical surface is added at the R(0.5±0.1)mm transition fillet to make the transition area between the nitrided and non-nitrided surfaces smoother. This improves the stress condition when machining the φ3.5mm outer circle on the coordinate grinder and eliminates the edge chipping phenomenon. Figures 4 and 5 show the geometric treatment of the φ3.6mm cylindrical fillet before and after process improvement, respectively. Figure 4. Geometric treatment of the φ3.6mm cylindrical fillet before process improvement
Figure 5. Geometric treatment of the φ3.6mm cylindrical fillet after process improvement
(3) Sealing groove deformation compensation: The edges of the sealing groove will shrink due to expansion deformation. This is caused by the sharp edge effect of the nitriding process. With high nitrogen concentration, the volume expansion deformation is greater than in other locations [9, 10]. The sealing groove with a width of 3.4mm and the retaining ring groove with a width of 1.1mm have large tolerances. The changes in groove width before and after nitriding are shown in Table 1. Based on this, the dimensional tolerances before nitriding are adjusted to ensure the final dimensional requirements. Table 1. Changes in Groove Width Before and After Nitriding (Unit: mm)
Table 1 shows that the change in groove width before and after nitriding for a 3.4mm groove is 0.026–0.035mm, thus determining that the groove width tolerance before nitriding should be compressed to 3.4mm; the change in groove width before and after nitriding for a 1.1mm groove is 0.010–0.027mm, thus determining that the groove width tolerance before nitriding should be compressed to 1.1mm. After compressing the tolerances, the final groove width dimensions of the parts are all within acceptable limits.
PART 7
Conclusion
Nitrided surfaces possess high hardness and high wear resistance, and their performance remains good at high temperatures. Therefore, nitrided stainless steel parts are widely used in aero-engine products. Machining nitrided surfaces is relatively difficult, requiring high standards for tool wear resistance, selection of cutting parameters, and planning of cutting paths. By increasing the protective conical transition fillet and compressing the dimensional tolerances before nitriding, the problems of low machining efficiency, easy chipping of edges, and reduced groove width after nitriding of stainless steel parts were solved, overcoming the machining difficulties of such parts. The pass rate of parts increased from about 50% before the improvement to 100%, achieving good results.
