What are the mechanical properties of precision steel pipe machining

In the precision machining of steel pipes, cutting is the core process that determines the dimensional accuracy, surface integrity, and service performance of the parts. The cutting process is essentially a dynamic mechanical interaction between the tool and the workpiece, accompanied by a series of mechanical phenomena such as dynamic changes in cutting force, the generation and conduction of cutting heat, plastic deformation of the material, and the evolution of surface layer mechanical properties. These mechanical properties not only directly affect the stability of the cutting process and tool life, but also determine the degree of work hardening, residual stress distribution, and microstructure of the precision steel pipe surface. Based on the technological characteristics of precision steel pipe machining, this paper systematically studies the generation mechanisms of core mechanical properties such as cutting force, cutting heat, work hardening, and residual stress, analyzes key influencing factors, and provides mechanical theoretical support for optimizing cutting parameters and improving machining quality.

Precision steel pipe machining involves multi-field coupling effects, and its mechanical properties exhibit significant dynamism and complexity. The core mechanical properties mainly include the dynamic distribution of cutting force, the generation and conduction of cutting heat, the formation of work hardening, and the evolution of residual stress. These properties are interrelated and mutually influential, jointly determining the quality and efficiency of machining.

(I) What are the characteristics and generation mechanism of the cutting force in precision steel pipes?

Cutting force is the force exerted by the cutting tool on the workpiece material during the cutting process. It is also the force required for the material to undergo plastic deformation, separation, and overcome friction. Its magnitude and distribution directly reflect the stability of the cutting process. According to the direction of action, cutting force can be divided into main cutting force (along the cutting speed direction), feed resistance (along the feed direction), and back force (perpendicular to the feed plane). These three constitute a spatial force system, and the magnitude and direction of their resultant force change dynamically with the cutting process. The generation of cutting force in precision steel pipes mainly stems from two aspects:

(a) The deformation resistance generated by the plastic deformation of the material, which is the main component of the cutting force. Under the squeezing action of the tool's rake face, the steel pipe material undergoes shear slip, forming chips. During this process, the interatomic bonding force is broken, and the resistance of lattice distortion and dislocation movement must be overcome.

(b) The frictional resistance between the tool, workpiece, and chips, including the sliding friction between the rake face and the chips, and the elastic friction between the flank face and the machined surface. For thin-walled precision steel pipes, due to the weak rigidity of the workpiece, the influence of the back force is particularly significant, easily leading to elastic deformation of the workpiece, which in turn causes dimensional deviations and surface ripples. Experimental data show that during the precision turning of 45# precision steel pipes, the main cutting force accounts for 60%-70% of the total cutting force, and the back force accounts for 20%-30%. When the cutting depth increases from 0.1mm to 0.3mm, the main cutting force can increase from 800N to 2200N, and the back force increases from 250N to 750N.

(II) What are the cutting thermal characteristics and generation? Mechanisms of precision steel pipes? Cutting heat is the product of the conversion of mechanical energy into thermal energy during the cutting process. Its generation and conduction directly affect tool life, workpiece thermal deformation, and surface layer mechanical properties. The generation of cutting heat is mainly concentrated in three regions:

(a) Shear deformation zone (first deformation zone): In this region, the material undergoes intense plastic deformation, and lattice distortion and dislocation movement generate a large amount of heat, accounting for 60%-70% of the total cutting heat;

(b) Tool-chip contact zone (second deformation zone): When the chip slides along the rake face, intense friction generates heat, accounting for 20%-30%.

(c) Tool-machined surface contact zone (third deformation zone): Elastic friction between the flank face and the machined surface, as well as the elastic recovery of the material, generates a small amount of heat, accounting for 5%-10%.

During precision steel pipe cutting, the heat transfer paths are mainly three: carried away by the chip, conducted through the tool, and conducted through the workpiece. Among these, the heat carried away by the chip accounts for the highest proportion (50%-80%), while the heat conducted through the tool and workpiece accounts for a relatively lower proportion. However, for precision steel pipe materials with poor thermal conductivity, cutting heat easily accumulates in the cutting area, leading to a rapid increase in temperature, reaching up to 800-1200℃. High temperatures trigger thermal and chemical wear of the cutting tool material, while simultaneously causing oxidation and microstructural phase transformation of the workpiece surface layer, exacerbating work hardening and residual tensile stress.

(III) What are the work hardening characteristics and mechanisms of precision steel pipes?

Work hardening (also known as cold work hardening) is a common surface layer mechanical property in the machining of precision steel pipes. It refers to the phenomenon where the machined surface layer material undergoes plastic deformation under cutting force, leading to lattice distortion, increased dislocation density, and consequently, a significant increase in surface layer hardness. The hardness, depth, and uniformity of the work-hardened layer directly affect the wear resistance and fatigue life of the parts. The core of work hardening is the plastic deformation and dislocation proliferation of the surface layer material during cutting. Under the squeezing and friction of the cutting tool, the machined surface layer material undergoes shear plastic deformation, the grains are elongated and refined, and a large number of dislocations are generated. These dislocations intertwine and block each other, resulting in enhanced resistance to further deformation and increased hardness. For precision steel pipes, the depth of the work-hardened layer is typically between 0.05 and 0.3 mm, and the hardness can be increased by 20%-50% compared to the base material. For example, after precision machining of 304 stainless steel precision pipes, the surface layer hardness can increase from HV200 of the base material to HV280-350. Excessive work hardening can lead to increased surface brittleness, making it prone to microcracks and reducing the service reliability of the parts.

(IV) What are the characteristics and generation mechanisms of residual stress in precision steel pipes?

Residual stress refers to the unbalanced internal stress remaining inside the workpiece after machining. Its distribution (tensile or compressive stress), amplitude, and depth directly affect the dimensional stability and fatigue life of the precision steel pipe. Based on its cause, cutting residual stress can be divided into thermal stress, plastic deformation stress, and phase transformation stress; the superposition of these three forms the final residual stress distribution.

Thermal stress originates from the non-uniform temperature field in the cutting zone during the cutting process: the material in the cutting zone expands due to heat, while the surrounding unheated material restricts this expansion, generating compressive stress; during cooling, the surface layer material contracts rapidly and is constrained by the internal material, ultimately forming residual tensile stress. Plastic deformation stress arises because the surface layer material undergoes plastic deformation under the cutting force, while the internal material does not. After plastic deformation, the surface layer material cannot fully recover and is constrained by the internal material, forming residual compressive stress. Phase transformation stress originates from the microstructure phase transformation of the surface layer material at high temperatures, resulting in stress generated by the constrained volume change before and after the phase transformation. In precision steel pipe machining, the coupling effect of thermal stress and plastic deformation stress is usually dominant. If thermal stress is dominant, residual tensile stress easily forms on the surface layer; if plastic deformation stress is dominant, residual compressive stress easily forms. For example, when 45# precision steel pipe is precision machined at low speed, plastic deformation stress dominates, and residual compressive stress (with an amplitude of -50 to 200 MPa) is formed on the surface layer; when precision machined at high speed, thermal stress dominates, and residual tensile stress (with an amplitude of 100 to 300 MPa) is formed on the surface layer.