surface-integrity-machining

name	surface-integrity-machining
description	Machined surface integrity — residual stress from machining (tensile vs. compressive), white layer formation (hardened steels), microstructural alterations (HAZ, phase transformation), work hardening (microhardness depth profile), surface roughness from machining (Ra, Rz, RSm, Rsk, Rku), cutting parameters effect on surface (feed rate, depth of cut, tool geometry), chip formation and BUE, built-up edge, tool wear effects, and CIRP recommendations for hard turning and milling.
metadata	{"priority":7,"promptSignals":{"phrases":["surface integrity","machining surface integrity","white layer","residual stress machining","hard turning","machined surface"],"minScore":3}}

Machined Surface Integrity — Complete Skill

Surface Integrity Overview

Definition (CIRP)

Surface integrity: the inherent or enhanced condition of a surface produced by machining or other surface generation processes Encompasses:

1. Surface topography (roughness, waviness, lay)
2. Mechanical alterations (residual stress, work hardening, microhardness)
3. Metallurgical alterations (white layer, phase transformation, grain deformation)

Functional importance: Fatigue strength: compressive residual stress → longer life; tensile → shorter life Wear resistance: microhardness, surface texture affect friction and wear Corrosion: residual stress accelerates SCC; surface damage creates corrosion initiation sites Dimensional stability: residual stress causes distortion if material is removed post-machining

Surface Roughness from Machining

Theoretical Roughness (Turning)

Feed mark profile — cusps from tool nose radius: Ra_theoretical = f² / (32 × r_ε) [mm; f = feed rate [mm/rev]; r_ε = tool nose radius [mm]] Rz_theoretical = f² / (8 × r_ε) [mm; Rz ≈ 4 × Ra_theoretical for this profile]

Example: f = 0.1 mm/rev; r_ε = 0.8 mm: Ra_theoretical = 0.01/(32×0.8) = 0.00039 mm = 0.39 μm Actual Ra typically 1.5–3× theoretical due to BUE, vibration, material side flow

Surface roughness parameters: Ra: arithmetic mean roughness (most commonly specified) Rz: average peak-to-valley height (5-point ISO definition) RSm: mean spacing of profile elements (tool marks spacing ≈ feed in turning) Rsk (skewness): positive = peaks; negative = valleys; machined surfaces often Rsk < 0 (valleys from cutting) Rku (kurtosis): > 3 = sharp peaks; < 3 = flat; ground surfaces Rku = 4–6

Achieving target Ra: Ra = 0.8 μm: f < 0.16 mm/rev (r_ε = 0.8 mm); fine machining Ra = 0.4 μm: f < 0.11 mm/rev or r_ε ≥ 1.6 mm Ra = 0.2 μm: precision turning; requires controlled conditions, sharp tools Ra = 0.1 μm: ultra-precision, SPDT (single point diamond turning) or fine grinding

Milling roughness: Ra_milling ≈ f_z² / (8 × r_ε) + a_e² / (8R²) × ... [f_z = feed per tooth; a_e = radial depth; R = cutter radius; complex formula for 3D case] Cusp height from ball-end milling: h_cusp = f_s²/(8R) [f_s = scallop spacing]

Residual Stress from Machining

Mechanisms

Two competing effects:

1. Thermal gradient: cutting heat → surface heats above bulk; upon cooling, surface contracts against bulk → tensile residual stress (dominant in high-speed cutting, worn tools)
2. Mechanical deformation: tool forces plastic deformation beneath surface → compressive residual stress induced in layer below (dominant in low-speed, sharp tool, heavy tool engagement)

Turning/milling steel residual stress profiles: Sharp tool, low cutting speed: σ_residual at surface = -200 to -500 MPa (compressive) Worn tool or high speed: σ_residual = +300 to +600 MPa (tensile) — detrimental for fatigue! Depth of significant residual stress: 0.05–0.5 mm

Influencing factors:

Factor	Effect on Surface Residual Stress
↑ Feed rate	More tensile (higher mechanical deformation → complex)
↑ Cutting speed	More tensile (thermal effect dominant)
↑ Tool nose radius	More compressive (increased burnishing effect)
Tool wear (flank wear)	More tensile (increased friction and heat)
Coolant	Less tensile (reduces temperature gradient)
↑ Depth of cut	Increases depth of affected layer; less effect on surface magnitude

Hard turning (PCBN on hardened steel, 55–65 HRC): Residual stress: typically compressive at surface (-200 to -800 MPa); depends on tool condition Tensile transition: stress becomes tensile at depth 0.05–0.2 mm (subsurface tensile hump) Fatigue implication: beneficial (compressive at surface) → hard turning can replace grinding for bearing races

White Layer

Formation and Properties

White layer (also "White Etching Layer"): phase transformation layer on machined surface; etches white in optical microscopy with Nital etchant Composition: untempered martensite (from rapid quench of austenite); nano-scale (<50 nm) grain structure Thickness: 1–50 μm (depends on cutting conditions)

Formation conditions: High flash temperature → austenite transformation → rapid quench by surrounding material → martensite Temperature at machined surface: 600–900°C briefly during cutting (thermocouple measurement) Also: severe plastic deformation at extremely high strain rates → mechanical white layer (different mechanism)

Properties of white layer: Hardness: 800–1,200 HV (much harder than base; 500 HV for 60 HRC steel) Brittle: fracture toughness very low; highly susceptible to cracking Tensile residual stress: usually associated with tensile surface stress

Effect on fatigue: White layer + tensile residual stress → fatigue crack initiation at/near surface Bearing steel (52100): white layer presence → reduced rolling contact fatigue life by 30–50% Rejection criteria: typically white layer < 2–5 μm for bearing components

Detection: Metallographic section + Nital etch (4% HNol); optical/SEM microscopy; EBSD for phase ID Eddy current (Barkhausen noise): correlates with microstructural changes non-destructively XRD: residual stress measurement identifies tensile zone associated with WL

Hard Turning White Layer Control

Reduce/eliminate white layer: Use sharp PCBN tool (prevent high friction heat) Low cutting speed: V_c < 120 m/min (reduce flash temperature) Cryogenic cooling (LN₂): dramatically reduces white layer Keep flank wear VB < 0.1 mm (worn tool → exponentially more heat)

Work Hardening

Microhardness Depth Profile

Hardness increases in surface layer due to: Plastic deformation → dislocation density increase → work hardening Effect: ΔHV = 15–50% above bulk for austenitic stainless; 5–15% for hardened steel

Microhardness profile: Peak at surface (or just below surface): HV_max = HV_bulk × (1 + WH_rate) Decreases exponentially to bulk: HV(z) = HV_bulk + (HV_max - HV_bulk) × exp(-z/λ_WH) λ_WH = work hardening decay depth: 0.05–0.5 mm depending on material and conditions

Measuring method: Vickers microhardness (500 gf or less) on polished cross-section at measured depths z from surface Minimum spacing: 3× indent diagonal between measurements; minimum 0.025 mm from surface practical limit

Functional effect: Compressive work hardening → reduces fatigue crack initiation rate (beneficial) Excessive deformation → micro-damage at grain boundaries → reduces ductility locally (detrimental) Austenitic stainless: heavy work hardening → HV from 180 to 350 → significant residual stress

Built-Up Edge (BUE)

BUE: workpiece material welds to rake face of tool at low-moderate cutting speeds Mechanism: adhesive bond at high contact pressures and temperatures; below β-transition temperature Effect on surface: BUE periodically breaks off → leaves rough surface (Ra increases 2–5×); damages workpiece surface BUE avoidance: V_c > critical (β-transition); use coated tools (TiN, TiAlN reduces adhesion); lubrication

BUE zone by material: Steel: BUE at V_c = 20–80 m/min; above: stable chip Aluminum: severe BUE at V_c = 50–200 m/min; above: long continuous chip Titanium: BUE tendency high at all speeds (adhesive); coatings help

Measurement Methods

Surface roughness: contact profilometer (ISO 4288, ISO 21920); non-contact: confocal microscopy, focus variation Residual stress: XRD sin²ψ (ISO/TS 21432); hole drilling ASTM E837; Barkhausen noise (qualitative) Microhardness: Vickers HV0.05 to HV0.5 (ASTM E92); minimum section taper-cut specimen White layer: optical microscope (500–1,000×); Nital etch; SEM-EBSD for phase confirmation

Standards and References

Standard	Scope
ISO 4287	Surface roughness: terms, definitions, parameters
ISO 4288	Assessment of surface texture
ASTM E92	Vickers hardness testing
CIRP Keynote (2002)	Surface integrity in machining — comprehensive review
M'Saoubi et al. (2008) CIRP Annals	Surface integrity in machining — state of the art
SAE AS9143	Nonconformance for surface integrity (aerospace)

Output

Provide: workpiece material (alloy; hardness HRC or HV; heat treatment; condition: annealed/hardened), machining operation (turning/milling/grinding; machine tool type; CNC control; coolant: dry/wet/MQL), cutting parameters (V_c [m/min]; f [mm/rev or mm/tooth]; a_p [mm]; a_e [mm]; tool: insert grade; nose radius [mm]; coating; wear state VB [mm]), theoretical Ra prediction (Ra_theoretical from feed+nose radius formula [μm]), expected actual Ra range [μm] (considering BUE, vibration, tool wear; achievable vs. required), residual stress prediction (surface σ_res: tensile/compressive; magnitude [MPa] from literature for similar conditions; dominant mechanism: thermal or mechanical; depth profile trend), white layer risk (temperature estimate at surface; WL risk: low/medium/high; VB check < 0.1 mm; recommended V_c < [m/min] if WL concern), work hardening (ΔHV estimate [%]; depth [μm]; beneficial or excessive; Rsk of surface profile), BUE check (operating speed vs. BUE zone for material; BUE avoidance: V_c or coating recommendation), measurement plan (Ra profilometer per ISO 4288; XRD for residual stress; microhardness cross-section for critical components), and applicable standard (ISO 4287; CIRP keynote; SAE AS9143 if aerospace).