Industry Professional Knowledge: Thermal Stress & Structural Stress in Steel Heat Treatment

In the field of metal material processing and heat treatment, thermal stress and structural stress, also known as phase transformation stress, are two fundamental internal stresses inevitably generated during steel heating and quenching processes. Their formation mechanisms, evolutionary rules, residual stress distribution and impact characteristics are completely different. These two types of stresses always superimpose and interact with each other, becoming the primary inducement for workpiece deformation, dimensional out-of-tolerance, warping and quenching cracking. Mastering their principles and control strategies is essential for heat treatment engineers, production technicians and quality management personnel in the steel manufacturing industry.

1. Comprehensive Introduction to Thermal Stress

1.1 Essential Nature & Generation Conditions

Thermal stress is essentially generated by uneven thermal expansion and cold contraction. When steel workpieces are heated or cooled, obvious temperature gradients form between the surface and the core, as well as between thick and thin sections. The metal parts with different temperatures produce inconsistent expansion or shrinkage deformation, and the mutual constraint of adjacent metal tissues hinders free deformation, thus generating internal thermal stress.

It mainly occurs in the processes of rapid heating, furnace charging and discharging, overall quenching cooling, and slow cooling of large-section workpieces.

1.2 Formation & Evolution Process (Quenching Cooling as Standard)

In the initial stage of quenching cooling, the workpiece surface contacts the cooling medium first, the temperature drops rapidly and the shrinkage amplitude is large; while the core cools slowly with small shrinkage. Restricted by the core, the surface is in a tensile stress state, and the core is in a compressive stress state.

In the later stage of cooling, the surface temperature tends to be stable and the structure is basically finalized. The core continues to cool and produce shrinkage, but its shrinkage deformation is constrained by the already rigid surface layer. Stress reversal occurs eventually, forming a stable residual stress field: the surface layer presents compressive stress, and the core presents tensile stress.

1.3 Main Influencing Factors of Thermal Stress

The magnitude of thermal stress is determined by multiple comprehensive factors: temperature difference gradient caused by cooling and heating rate, workpiece cross-sectional size and wall thickness difference, thermal conductivity of steel grade, linear thermal expansion coefficient, elastic modulus of the material, as well as the heating and cooling temperature interval and holding time. Larger temperature difference and faster temperature change speed will significantly increase thermal stress.

2. Comprehensive Introduction to Structural (Phase Transformation) Stress

2.1 Essential Nature & Generation Conditions

Structural stress originates from specific volume difference during solid-state phase transformation of steel. Different metallographic structures of steel have different inherent specific volumes. The specific volume order of common steel phases is fixed: Austenite < Ferrite / Pearlite < Bainite < Martensite.

When austenite transforms into martensite during quenching, the volume expands obviously. Due to the asynchronous phase transformation between the surface and the core of the workpiece, inconsistent volume expansion is constrained, producing phase transformation structural stress.

2.2 Formation & Evolution Process (Quenching Cooling as Standard)

During quenching, the workpiece surface first cools down to the Martensite start temperature (Ms point), completes austenite-martensite transformation preferentially, and produces volume expansion. At this time, the core is still maintained in the austenite state without phase change and volume change.

Later, the core gradually cools to the Ms point and undergoes martensitic transformation and volume expansion. The surface tissue that has completed transformation in advance is pulled and constrained by the expanding core. Finally, the residual stress distribution is formed as: the surface layer is tensile stress, and the core is compressive stress, which is completely opposite to the distribution rule of thermal stress.

2.3 Main Influencing Factors of Structural Stress

The key influencing factors include carbon content of steel — higher carbon content leads to larger specific volume of martensite and greater structural stress; specific volume difference before and after phase transformation; asynchronism of phase transformation affected by cooling speed and workpiece section size; material hardenability, Ms point temperature, and alloy composition of steel. Alloy elements will change the Ms point and phase transformation process, thus affecting the magnitude of structural stress.

3. Detailed Comparative Analysis of Thermal Stress and Structural Stress

表格

Comparison Dimension Thermal Stress Structural (Transformation) Stress

Fundamental Cause Uneven temperature field leads to inconsistent thermal expansion and contraction Solid-state phase transformation leads to different specific volume and asynchronous volume change

Residual Stress Distribution Surface compressive stress, Core tensile stress Surface tensile stress, Core compressive stress

Dominant Application Scenario Large cross-section workpieces, slow cooling process, normalizing and annealing Small thin workpieces, rapid quenching process, high hardenability steel

Main Defect Inducement Prone to thermal crack, overall warping and macroscopic deformation Prone to quenching cold crack, local splitting and delayed cracking

Core Mechanism Summary Generated by uneven cold and heat Generated by asynchronous phase transformation

4. Superposition Effect, Engineering Hazards & Practical Significance

In actual heat treatment production, thermal stress and structural stress do not exist independently; they coexist all the time and superimpose on each other to form the total internal stress of the workpiece.

For small and thin workpieces with fast cooling speed, structural stress is in the dominant position; for large thick workpieces with slow overall cooling speed, thermal stress becomes the main controlling stress.

Main Engineering Hazards

Dimensional deformation: When the superimposed internal stress exceeds the yield strength of the material, the workpiece produces irreversible plastic deformation, resulting in dimensional out-of-tolerance and shape distortion.

Cracking failure: Excessive thermal stress induces thermal cracks at high temperature; excessive structural stress induces cold cracks after quenching, which is the main reason for workpiece scrap in heat treatment.

Performance attenuation: Long-term residual internal stress reduces the fatigue strength, impact toughness and corrosion resistance of parts, and causes dimensional instability during long-term service.

Industrial Practical Significance

Reasonably controlling thermal stress and structural stress can effectively reduce the scrap rate of heat treatment, improve the dimensional accuracy and service life of parts, and is widely applied in automobile parts, engineering machinery, bearing tools, mold steel and other manufacturing industries.

5. Industrial Standard Control Key Points & Process Optimization Measures

Cooling Process Optimization

Adopt graded quenching and isothermal quenching technology, properly reduce the quenching cooling rate, narrow the temperature gradient between surface and core, and reduce the time difference of phase transformation, so as to weaken both thermal stress and structural stress.

Heating and Furnace Loading Control

Adopt preheating and step-by-step temperature rise process to avoid rapid heating; optimize the furnace loading mode to ensure uniform heating of workpieces, prevent local overheating and excessive temperature difference.

Material and Structural Design Optimization

For high-carbon and high-alloy steel, properly reduce quenching cooling intensity; avoid sharp corners, grooves and abrupt thickness changes in workpiece design to eliminate stress concentration sources from the structure.

Timely Stress Relief Treatment

Timely tempering after quenching is the most effective process measure to eliminate residual internal stress, balance stress distribution and prevent delayed quenching cracks.

Reasonable Matching of Steel Grade and Process

Select steel grades with appropriate hardenability according to workpiece size, match corresponding heat treatment processes, control the phase transformation range and cooling rate, and fundamentally inhibit the excessive generation of internal stress.