Full Hard Temper: Maximum Hardness State in Cold Rolled Steel

Definition and Basic Concept

Full Hard Temper refers to the maximum hardness and strength condition achieved in cold-rolled steel through extensive cold reduction without subsequent annealing or heat treatment. It represents the highest level of work hardening that can be practically achieved in commercial steel processing, typically characterized by high yield strength, reduced ductility, and increased springback properties.

Full Hard Temper is a critical designation in the steel industry that indicates a specific mechanical property profile resulting from severe plastic deformation during cold rolling. This condition is particularly important in applications requiring high strength, dimensional stability, and wear resistance without additional heat treatment processes.

In the broader context of metallurgy, Full Hard Temper represents an extreme state in the spectrum of work-hardened conditions, contrasting with annealed, quarter-hard, half-hard, and three-quarter-hard tempers. It exemplifies how mechanical processing alone can dramatically alter material properties through microstructural modification without changing chemical composition.

Physical Nature and Theoretical Foundation

Physical Mechanism

At the microstructural level, Full Hard Temper results from severe plastic deformation that introduces a high density of dislocations within the crystal lattice. These dislocations interact and entangle with each other, creating barriers to further dislocation movement and thereby increasing the material's resistance to deformation.

The cold rolling process flattens and elongates the grains in the rolling direction, creating a preferred crystallographic orientation (texture) and increasing the total grain boundary area. This grain refinement contributes significantly to strengthening through the Hall-Petch relationship, where smaller grain sizes yield higher strength.

Strain hardening in Full Hard Temper steel also involves the formation of deformation twins and stacking faults, particularly in steels with lower stacking fault energy. These defects further impede dislocation movement, contributing to the exceptional hardness and strength characteristic of this temper condition.

Theoretical Models

The primary theoretical model describing Full Hard Temper is the strain hardening (work hardening) model, mathematically expressed through the Hollomon equation. This power law relationship connects true stress to plastic strain and has been fundamental to understanding work hardening since the 1940s.

Historically, understanding of work hardening evolved from empirical observations in the 19th century to dislocation theory in the mid-20th century. Early metallurgists noted the phenomenon but lacked the theoretical framework to explain it until electron microscopy revealed dislocation structures.

Alternative theoretical approaches include the Voce equation, which better describes saturation hardening behavior at high strains, and the Kocks-Mecking model, which incorporates dislocation density evolution. These models provide complementary perspectives on the work hardening phenomenon underlying Full Hard Temper.

Materials Science Basis

Full Hard Temper directly relates to crystal structure through dislocation density and arrangement. In body-centered cubic (BCC) iron, dislocations interact differently than in face-centered cubic (FCC) phases, affecting how the material responds to cold working and ultimately determining maximum achievable hardness.

Grain boundaries in Full Hard Temper steel become elongated and aligned with the rolling direction, creating anisotropic mechanical properties. These boundaries act as barriers to dislocation movement, contributing significantly to the material's strength through Hall-Petch strengthening.

The fundamental materials science principle of strain energy storage underlies Full Hard Temper. Cold rolling introduces substantial stored energy in the form of crystal defects, creating a thermodynamically unstable state that provides the driving force for recrystallization if the material is subsequently heated.

Mathematical Expression and Calculation Methods

Basic Definition Formula

The Hollomon equation represents the fundamental relationship governing work hardening in Full Hard Temper steel:

$$\sigma = K\varepsilon^n$$

Where $\sigma$ is the true stress, $K$ is the strength coefficient (material constant), $\varepsilon$ is the true plastic strain, and $n$ is the strain hardening exponent (typically 0.05-0.15 for Full Hard steel).

Related Calculation Formulas

The reduction in thickness required to achieve Full Hard Temper can be calculated using:

$$r = \frac{t_0 - t_f}{t_0} \times 100\%$$

Where $r$ is the percent reduction, $t_0$ is the initial thickness, and $t_f$ is the final thickness. Full Hard Temper typically requires reductions of 60-80%.

The relationship between hardness and tensile strength for Full Hard steel can be approximated by:

$$UTS \approx k \times HV$$

Where $UTS$ is ultimate tensile strength (MPa), $HV$ is Vickers hardness, and $k$ is a correlation factor (typically 3.0-3.5 for Full Hard steel).

Applicable Conditions and Limitations

These formulas apply primarily to low and medium carbon steels with carbon content below 0.3%. For higher carbon or highly alloyed steels, the relationships become more complex and may require empirical determination.

The Hollomon equation assumes uniform deformation and becomes less accurate at very high strain levels where strain localization occurs. It also does not account for strain rate sensitivity or temperature effects during deformation.

These mathematical models assume continuous cold rolling without intermediate annealing. Any recovery or recrystallization processes will invalidate these relationships and require recalibration of the model parameters.

Measurement and Characterization Methods

Standard Testing Specifications

ASTM A370: Standard Test Methods and Definitions for Mechanical Testing of Steel Products - Covers tensile testing procedures for determining mechanical properties of Full Hard steel.

ASTM E18: Standard Test Methods for Rockwell Hardness of Metallic Materials - Specifies hardness testing procedures commonly used to verify Full Hard Temper.

ISO 6892-1: Metallic materials — Tensile testing — Part 1: Method of test at room temperature - Provides international standards for tensile testing applicable to Full Hard steel characterization.

Testing Equipment and Principles

Universal testing machines with load capacities of 50-300 kN are typically used for tensile testing of Full Hard steel. These machines measure force and displacement to generate stress-strain curves that reveal key mechanical properties.

Hardness testers (Rockwell, Vickers, or Brinell) measure material resistance to indentation. Rockwell hardness testing (typically scale C or B) is most common for rapid verification of Full Hard Temper in production environments.

Optical and electron microscopy equipment enables microstructural characterization of grain structure, dislocation density, and texture development. Advanced techniques like EBSD (Electron Backscatter Diffraction) can quantify crystallographic texture characteristic of Full Hard Temper.

Sample Requirements

Standard tensile specimens follow ASTM E8/E8M dimensions, typically with gauge lengths of 50mm and cross-sectional areas appropriate for the material thickness. For thin sheet, subsize specimens may be used.

Surface preparation for hardness testing requires flat, clean surfaces free of scale, oxide, or decarburization. For thin materials, proper backing support is essential to prevent deflection during testing.

Specimens must be cut with their axis either parallel or perpendicular to the rolling direction, with clear documentation of orientation due to the anisotropic properties of Full Hard steel.

Test Parameters

Testing is typically conducted at room temperature (23±5°C) under controlled humidity conditions to ensure reproducibility. For specialized applications, testing at elevated or cryogenic temperatures may be required.

Standard tensile testing employs strain rates of 0.001-0.008 s⁻¹ in the elastic region, with potential increases after yielding. Consistent strain rates are critical as Full Hard materials can exhibit strain rate sensitivity.

Hardness testing parameters include standardized loads (typically 150 kgf for Rockwell C scale) and dwell times (10-15 seconds) to ensure consistent results across different testing locations.

Data Processing

Stress-strain curves are generated from force-displacement data, with yield strength typically determined using the 0.2% offset method due to the absence of a distinct yield point in Full Hard steel.

Statistical analysis typically involves multiple specimens (minimum of three) with calculation of mean values and standard deviations. Outlier analysis may be performed according to ASTM E178 guidelines.

Hardness conversion between different scales (Rockwell, Brinell, Vickers) uses standardized conversion tables in ASTM E140, though these conversions have increased uncertainty for Full Hard materials.

Typical Value Ranges

Steel Classification	Typical Value Range	Test Conditions	Reference Standard
Low Carbon Steel (1008-1010)	85-95 HRB, 550-650 MPa UTS	Room temperature, 60-80% reduction	ASTM A109
Medium Carbon Steel (1045)	25-35 HRC, 800-950 MPa UTS	Room temperature, 60-75% reduction	ASTM A108
HSLA Steel	90-100 HRB, 700-850 MPa UTS	Room temperature, 65-75% reduction	ASTM A1011
Stainless Steel (304)	35-42 HRC, 1300-1500 MPa UTS	Room temperature, 60-70% reduction	ASTM A666

Carbon content significantly affects the maximum hardness achievable in Full Hard Temper, with higher carbon steels reaching higher hardness values but with increased risk of cracking during processing.

These values represent typical ranges for commercial production; actual values may vary based on precise chemical composition, processing history, and measurement methods. Higher alloyed steels generally achieve higher strength at Full Hard Temper.

A consistent trend across steel types is the significant increase in yield strength accompanied by substantial reduction in elongation (typically below 5%) when processed to Full Hard Temper.

Engineering Application Analysis

Design Considerations

Engineers must account for the high springback characteristic of Full Hard steel, often requiring overbending of 15-25% in forming operations or specialized tooling designed specifically for these materials.

Safety factors of 1.5-2.0 are typically applied when designing with Full Hard steel due to its reduced ductility and limited capacity to redistribute stresses through plastic deformation before failure.

Material selection decisions involving Full Hard Temper often prioritize strength and wear resistance over formability, making it suitable for applications where parts are formed first in a softer state and then cold rolled to final hardness.

Key Application Areas

The automotive industry extensively uses Full Hard steel for safety-critical components like door impact beams, seat frames, and reinforcement brackets where high strength-to-weight ratio is essential for crash performance and fuel efficiency.

Electrical applications rely on Full Hard electrical steel (silicon steel) for transformer laminations and motor cores, where specific magnetic properties combined with high strength enable efficient energy conversion while withstanding electromagnetic forces.

Consumer goods manufacturing utilizes Full Hard stainless steel for appliance components, cutlery blanks, and razor blades where exceptional hardness provides wear resistance and edge retention while maintaining corrosion resistance.

Performance Trade-offs

Strength and ductility exhibit a classic inverse relationship in Full Hard steel, with the exceptional strength coming at the cost of reduced formability and elongation, limiting complex forming operations after hardening.

Fatigue resistance and impact toughness often conflict in Full Hard materials, as the high dislocation density that provides strength also reduces the material's ability to absorb energy during impact loading.

Engineers balance these competing requirements by selectively using Full Hard steel in components where its high strength is critical, while employing softer tempers or different materials in areas requiring greater ductility or impact resistance.

Failure Analysis

Brittle fracture represents the most common failure mode in Full Hard steel components, characterized by minimal plastic deformation before crack propagation and often initiated at stress concentrations or material defects.

The failure mechanism typically involves limited crack blunting due to restricted dislocation mobility, allowing cracks to propagate rapidly once initiated, particularly under tensile or bending loads perpendicular to the rolling direction.

Mitigation strategies include careful design to minimize stress concentrations, proper alignment of the material's rolling direction with principal stress directions, and in some cases, stress relief treatments to reduce residual stresses without significantly softening the material.

Influencing Factors and Control Methods

Chemical Composition Influence

Carbon content has the most significant impact on achievable hardness in Full Hard Temper, with each 0.1% increase in carbon typically raising maximum hardness by 3-5 HRC points while reducing maximum practical cold reduction.

Manganese enhances hardenability and contributes to solid solution strengthening, allowing Full Hard Temper to achieve higher strength levels, particularly in low carbon steels where it compensates for limited carbon hardening.

Trace elements like phosphorus and nitrogen can dramatically increase work hardening rates and maximum achievable hardness, but may also increase brittleness and susceptibility to cracking during cold rolling operations.

Microstructural Influence

Finer initial grain sizes accelerate work hardening during cold rolling, allowing Full Hard Temper to be achieved with less reduction but potentially limiting the maximum achievable strength due to earlier onset of instability.

Phase distribution significantly affects Full Hard properties, with ferritic-pearlitic structures behaving differently than martensitic or bainitic structures during cold rolling due to differences in dislocation mobility and work hardening behavior.

Non-metallic inclusions act as stress concentrators in Full Hard steel, potentially initiating premature failure and limiting achievable reduction before cracking occurs, making clean steelmaking practices essential for high-performance applications.

Processing Influence

Annealing treatments prior to cold rolling determine the starting microstructure and significantly influence work hardening behavior, with fully annealed structures typically allowing greater total reduction before reaching Full Hard Temper.

Rolling reduction schedule affects texture development and anisotropy in Full Hard steel, with single-pass heavy reductions producing different property profiles than multiple lighter passes to the same total reduction.

Cooling rates between rolling passes influence recovery processes, with faster cooling preserving dislocation structures and maintaining work hardening efficiency in subsequent passes, particularly important for achieving consistent Full Hard properties.

Environmental Factors

Elevated temperatures significantly reduce the strength of Full Hard steel through recovery and recrystallization processes, with noticeable softening beginning around 200°C for carbon steels and potentially lower for metastable stainless steels.

Hydrogen embrittlement susceptibility increases dramatically in Full Hard steel due to high internal stresses and dislocation density, making careful control of pickling processes and environmental exposure critical for maintaining mechanical integrity.

Cyclic temperature fluctuations can lead to dimensional instability in Full Hard components due to gradual relief of residual stresses, particularly important in precision applications like measuring instruments or gauges.

Improvement Methods

Grain refinement through controlled rolling and cooling prior to final cold reduction enhances both strength and toughness in Full Hard steel through Hall-Petch strengthening mechanisms while maintaining acceptable ductility.

Skin-passing (light cold rolling at 0.5-2% reduction) after achieving Full Hard Temper can improve surface finish, flatness, and yield strength while minimizing the impact on overall hardness and ductility.

Microalloying with small additions of elements like niobium, titanium, or vanadium can enhance grain refinement and precipitation strengthening, allowing Full Hard Temper to achieve higher strength levels without increasing brittleness.

Related Terms and Standards

Related Terms

Temper Rolling refers to light cold rolling (typically 0.5-5% reduction) applied to control flatness, surface finish, and mechanical properties, distinct from the heavy reduction (60-80%) used to achieve Full Hard Temper.

Work Hardening Exponent (n-value) quantifies a material's ability to distribute strain during deformation, with Full Hard materials exhibiting very low n-values (typically <0.10) compared to annealed materials (0.20-0.25).

Bauschinger Effect describes the phenomenon where prior deformation in one direction reduces yield strength during subsequent deformation in the opposite direction, particularly relevant when forming Full Hard materials under cyclic loading.

These terms are interconnected through their relationship to dislocation behavior and plastic deformation mechanisms, with Full Hard Temper representing an extreme case of work hardening where dislocation density approaches saturation.

Main Standards

ASTM A109/A109M "Standard Specification for Steel, Strip, Carbon (0.25 Maximum Percent), Cold-Rolled" defines Full Hard Temper as Temper 4, with specific hardness and tensile requirements for various steel grades.

EN 10139 "Cold rolled uncoated low carbon steel narrow strip for cold forming - Technical delivery conditions" provides European specifications for Full Hard (CR4) temper with corresponding mechanical property requirements.

JIS G 4051 "Carbon steels for machine structural use" differs from ASTM standards by emphasizing hardness ranges rather than minimum values for defining Full Hard Temper, particularly for spring steel applications.

Development Trends

Advanced high-strength steel (AHSS) development is exploring combinations of work hardening and transformation-induced plasticity to achieve Full Hard-level strengths with improved formability through carefully controlled multiphase microstructures.

Non-destructive evaluation technologies using magnetic Barkhausen noise and ultrasonic velocity measurements are emerging as production-friendly methods to verify Full Hard Temper without destructive testing.

Computational modeling of work hardening behavior using crystal plasticity finite element methods is advancing our ability to predict Full Hard properties from processing parameters, potentially enabling more precise control of final properties through optimized rolling schedules.