Heat Treatment of 4140 Steel: Hardening, Tempering and Stress Relieving


Heat treatment is critical for developing the required strength, hardness, and service properties in 4140 alloy steel. Proper control of heating and cooling cycles transforms the microstructure and mechanical attributes to meet design specifications. This article provides in-depth guidelines on effective heat treating methods including austenitizing, quenching, tempering, and stress relieving of 4140 steel.

We will examine how parameters like temperature, time, quenchant severity, and sequence are optimized based on section size, hardness targets, and final service stresses. Following established protocols for 4140 steel heat treatment ensures achieving the necessary combination of hardness, ductility, toughness, and fatigue resistance.

Overview of Heat Treating 4140 Steel

Heating and cooling under controlled conditions modifies 4140 steel on a metallurgical level by:

  • Altering phase proportions and grain sizes
  • Affecting carbide precipitation and transformation hardening
  • Removing residual stresses
  • Enhancing mechanical properties like strength, ductility, and hardness

4140 steel is particularly responsive to heat treating owing to its alloying additions which provide hardenability. This enables full through-hardening of sections via quenching and tempering processes.

Common heat treatments include:

  • Hardening – heating and rapid cooling to form martensite
  • Tempering – reheating to refine martensite and relieve stresses
  • Annealing – slow cooling to soften and improve machinability
  • Normalizing – controlled cooling to refine grain size
  • Stress relieving – heating to remove residual stresses

Now let’s look at the specific parameters and procedures for key heat treatment processes applied to 4140 steel.

Hardening 4140 Steel

Hardening strengthens and hardens 4140 steel by heating into the austenite region followed by rapid quenching to form a very hard martensitic structure:

Steps in Hardening

  • Heat uniformly to austenitizing temperature of 1525-1650°F
  • Soak until fully austenitized; soak times per section thickness
  • Quench rapidly in oil or water to form martensite
  • Quench severity tailored to desired hardness and section size

Austenitize Soak Time

  • Minimum 15 minutes when heating in vacuum or inert gas furnace
  • 1 hour per inch of thickness in air atmosphere
  • Ensure uniform temperature prior to quenching

Quenchant Selection

  • Moderate quench oils between 100-180°F for most applications
  • Faster water quenching for maximum hardness in thick sections
  • Brine or caustic quenches for extreme hardness

Quench Agitation and Flow

  • Agitate or flow quenchant across steel surface
  • Maintains vapor film for faster, uniform heat extraction

Quality Validation

  • Check hardness and microstructure to confirm
  • Evaluate hardness traverse on cross section

Rehardening 4140 Steel

If earlier heat treatment results are unacceptable, 4140 steel can be successfully rehardened by:

  • First softening via annealing followed by re-austenitizing and quenching
  • Reaustenitizing by quickly heating into hardening range
  • Always temper immediately after any rehardening of 4140 steel to prevent brittleness

Proper procedures ensure 4140 steel is fully hardened to obtain the optimal combination of hardness and strength required for service loads and use conditions.

Tempering of Hardened 4140 Steel

Tempering after quenching is vital for balancing needed hardness with ductility and toughness:

Tempering Objectives

  • Reduce brittleness of as-quenched martensite
  • Improve ductility and fracture toughness
  • Reduce internal stresses

Tempering Temperature Ranges

  • 375-725°F depending on desired hardness
  • Higher hardness requires lower tempering temperatures

Tempering Soak Times

  • 2 hours for plates and thick sections
  • 1 hour minimum for smaller parts

Tempering Cycles

  • Double or triple tempering achieves uniform properties
  • Cool back to room temperature between cycles

Quality Validation

  • Verify tempering via hardness testing and metallography
  • Impact and tensile testing confirms mechanical properties

Precise tempering maximizes 4140 steel hardness while controlling its brittleness. Temper immediately after quenching while steel is still hand-warm.

Stress Relieving 4140 Steel

Stress relieving is performed after manufacturing steps like welding, machining, grinding, or cold working:

Goals of Stress Relieving

  • Remove harmful tensile residual stresses
  • Prevent warpage or distortion
  • Avoid delayed cracking in service or during subsequent processing

Stress Relief Parameters

  • Hold at 1100-1250°F for 1 hour per inch of thickness
  • May require longer times for complex shapes
  • Slow furnace cool at 40°F/hour or slower

Quality Validation

  • Hardness check for unintended softening
  • Dimensional inspection confirms part stability
  • Strain gauge testing can map residual stresses

Stress relieving produces dimensional stability by removing residual stresses locked in from prior mechanical or thermal processing of 4140 steel. It prevents delayed cracking or distortion in service.

Annealing 4140 Steel

Full annealing involves very slow cooling from 1500-1600°F to obtain maximum softness:

Purposes of Annealing

  • Produce coarse, equiaxed ferrite+pearlite structure
  • Optimize machinability of difficult to machine shapes
  • Maximum ductility and formability for fabricating

Annealing Process

  • Heat to just above Ac3 and soak for carbide dissolution
  • Cool very slowly in furnace at 40-50°F per hour
  • Protect from drafts during cooling


  • Not used prior to hardening since it coarsens grain size
  • Only if starting material too hard to readily machine
  • Requires renormalizing and hardening after annealing

Annealing fully softens 4140 steel for enhanced machinability when fabricating complex or severe shapes. Dimensional changes during annealing must be accounted for.

Normalizing 4140 Steel

Normalizing is used prior to hardening to refine the grain size:

Purpose of Normalizing

  • Produces uniform fine-grained structure
  • Improves hardening response and properties
  • Removes effects of hot or cold working
  • Internal residual stresses are relieved

Normalizing Process

  • Heat to 1650-1700°F and equalize temperature
  • Cool in still air away from drafts
  • Cooling rate faster than annealing but slower than hardening


  • Often performed after hot forming like forging or bending
  • Use before machining to minimize distortion
  • Always normalize prior to austenitizing 4140 steel for hardening

Normalizing optimizes grain size, integrity, and homogeneity to maximize the hardening capability of 4140 steel for superior strength and toughness.

Proper heat treatment tailored to the steel composition, section size, and service stresses enables developing the required balance of hardness, strength, ductility, and toughness in 4140 steel components. Close monitoring of process parameters and validation testing ensures heat treat quality and integrity.

Best Practices for Hardening 4140 Steel

Hardening and quenching are critical processes for developing high strength in 4140 alloy steel. Here are proven guidelines for success:

  • Always normalize prior to hardening to refine grain size
  • Slow heat to 1450°F to allow alloy carbides to dissolve before rapid heating
  • Heat quickly from 1450°F up to the final austenitizing temperature
  • Soak uniformly at austenitizing temperature; use one hour per inch minimum
  • Use pyrometers and multiple thermocouples to verify temperature uniformity
  • Quench immediately in hot oil around 150°F for moderate cooling rate
  • Agitate oil quenchant to maximize heat transfer
  • For maximum hardness, substitute high speed water quenching instead of oil
  • Validate hardness achieved through testing on properly prepared samples
  • Temper immediately while steel is still hand-hot to prevent cracking

Careful heating, soaking, and quenching operations enable developing the full hardening capability in 4140 steel needed for reliable performance in demanding service conditions.

How to Select Optimal Quenchant for Hardening 4140 Steel

The quenching media and severity significantly affect the hardness achieved when hardening 4140 steel:


  • Most common quenchant for moderate cooling
  • Usually heat to 120-180°F before quenching
  • Fine microstructure with good toughness


  • Provides fastest cooling for maximum hardness
  • Can cause distortion and cracking in thick sections
  • Produces very fine martensite and high hardness


  • Accelerated cooling from salt lowers quench temperatures further
  • Increases risk of cracking
  • Only for small sections needing extreme hardness

Forced Air

  • Moderate cooling for flat or thin parts
  • Avoids distortion of fragile sections
  • Provides medium hardness capability

Polymer Quenchants

  • Intermediate cooling rate between oil and water
  • Reduced distortion and thermal shock versatility

The proper quenching method balances hardness desired against distortion tolerance for specific 4140 steel parts and applications. Faster quenching risks cracking while slower cooling reduces attainable hardness.

Tempering Treatment to Balance Hardness and Toughness of 4140 Steel

After quenching to full hardness, tempering is required to impart needed ductility and toughness to hardened 4140 steel:

Tempering Temperature Range

  • 375-700°F depending on required hardness
  • Higher hardness requires lower tempering temperatures

Holding Time

  • Minimum 2 hours for plates and thick sections
  • 1 hour minimum for smaller parts

Tempering Cycles

  • Double or triple tempering achieves uniform properties
  • Cool to room temperature between cycles

Expected Effects

  • Hardness decreases incrementally as tempering temperature rises
  • Toughness and ductility increase at expense of hardness
  • Removes brittleness of untreated martensite

Proper tempering produces the optimal balance of hardness and strength for service loads along with the ductility necessary to withstand shock and fatigue stresses.

Preventing Cracking When Hardening 4140 Steel

Susceptibility to quench cracking during the rapid cooling while hardening 4140 steel can be minimized:

  • Normalize prior to hardening to refine grain size
  • Pre-heat to 1200-1250°F to equalize temperatures
  • Select quenchant and agitation rate to avoid localized hard spots
  • Modify quench severity for specific steel composition and thickness
  • Use interrupted quenching with tempered martensite formation
  • Adjust austenitizing temperatures based on hardenability calculations
  • Add low temperature tempering immediately after quenching
  • Use compression stress inducing shot peening after quenching
  • Increase fracture resistance with minor alloy adjustments
  • Detect and repair cracks through penetrant testing after hardening

Controlling thermal and transformation stresses during quenching reduces the risk of cracking. Proper fixturing also minimizes part restraint. Close monitoring of process response provides feedback to avoid hardness-related cracking.

Main Causes of Distortion During Hardening of 4140 Steel

Several factors inherent to the rapid heating and cooling cycles used for hardening 4140 can lead to part distortion:

  • Uneven heating or cooling causes non-uniform dimensional changes
  • Restrained conditions prevent free thermal expansion and contraction
  • Section size differences create localized shrinkage differences
  • Phase transformations generate heat-affected zone property gradients
  • Relaxation of prior residual stresses from machining or cold work
  • Temperature gradients through thickness produce uneven thermal strains
  • Non-uniform quenchant flow and agitation
  • Dissimilar geometry causes non-uniform heat transfer rates
  • Carburized cases create steep carbon composition gradients
  • Impurities like sulfur and phosphorus segregate during heating

Careful fixturing, thermal equalization, uniform quenching, controlled transformation phasing, and stress relieving are vital for minimizing part distortion when hardening 4140 steel components. Thermal process simulation helps optimize the hardening process.

How to Design Fixtures to Minimize Distortion During 4140 Steel Hardening

Fixture design considerations for minimizing distortion when heat treating 4140 steel include:

  • Allow free expansion and contraction in the direction of largest surfaces
  • Orient parts to minimize thermal gradients during heating and cooling
  • Avoid excessive constraint which magnifies thermal stresses
  • Use compliant, cushioned supports to reduce friction
  • Incorporate weighting or springs to apply counteracting forces
  • Design flexure joints into fixtures to relieve stresses gradually
  • Allow clear airflow around parts for uniform gas circulation
  • Include an expansion allowance zone around parts
  • Incorporate insulation blocks to slow localized heat loss
  • Use fixtures made from materials with similar expansion coefficients
  • Enable quick, easy part transfer into quenchant with minimum handling

Through FEA analysis of the thermal and transformational stresses, fixtures can be optimized to minimize distortion, bowing, twisting, and cracking during 4140 steel hardening.

Importance of Temperature Uniformity When Hardening 4140 Steel

Achieving temperature uniformity throughout 4140 steel parts during austenitizing is vital to avoid distortion:

Temperature Differentials

  • Cause localized expansion/contraction differences
  • Result in unequal phase transformation rates


  • Can produces cracks in vulnerable areas
  • Leads to part distortion after quenching
  • Creates localized property variations


  • Use multiple thermocouples to identify hot and cold zones
  • Optimize part loads and fixture spacing
  • Allow sufficient time for temperature to equalize
  • Adjust heating rates and patterns based on data
  • Air quench initially if needed to normalize temperatures

Validation Methods

  • Temperature recording charts
  • Thermocouple grids and imaging
  • Automated thermal process control

Avoiding thermal gradients is imperative for saturation austenitizing withoutHot spots or cold spots. This minimizes distortion and property deviations.

Problems Caused by Non-Metallic Inclusions in 4140 Steel

Non-metallic oxide inclusions and sulfides in 4140 steel can lead to processing issues and degraded properties if not controlled:

Forging Problems

  • Inclusions deform into stringers causing cracks
  • Lower hot ductility and rupture risks

Fatigue Strength Reduction

  • Hard brittle inclusions act as initiators of fatigue cracks
  • Lowers endurance limit and component life

Machining Challenges

  • Hard particles cause excessive tool wear
  • Results in poor finish and reduced output

Poor Weldability

  • Oxides and sulfides link up causing weld cracks
  • Worsens weld impact toughness

Inferential Hardening Response

-Particles hinder heat flow, cause localized soft spots

  • Ruins properties of heat treated components

Corrosion Risks

  • Galvanic corrosion cells form around non-metallic inclusions
  • Accelerates localized pitting type corrosion damage

Stringent melt quality control and inclusion shape control minimizes these defects. Filtering, magnetic separation, and slag removal are also used to achieve cleaner 4140 steel.

Effects of Major Alloying Elements in 4140 Steel

The principal alloying elements in 4140 steel provide targeted benefits to its properties:


  • Primary hardenability agent
  • Increases hardenability and tensile strength
  • Enables deeper through-hardening


  • Secondary hardenability enhancer
  • Elevates creep strength and toughness
  • Maintains hardness at high tempering temperatures


  • Key deoxidizer for clean steel
  • Boosts low temperature toughness
  • Enhances quench crack resistance


  • Adds toughness and ductility
  • Refines ferrite grain size
  • Provides incremental hardenability


  • Deoxidizes and boosts creep resistance
  • Adds strength without compromising ductility
  • Improves flow stress

The synergy between carefully balanced alloying additions enables 4140 steel to offer an exceptional combination of fabricability, hardenability, strength, and toughness.

Target Core Hardness Levels for Heat Treated 4140 Steel

The intended service stresses and loads dictate the target core hardness levels when heat treating 4140 steel:

Light Loading

  • Surface hardness only required
  • Core hardness of 30-35 HRC may suffice

Low to Medium Stresses

  • Through-hardness of 36-40 HRC recommended
  • Prevents failure from stress raisers

High Cyclic Stresses

  • Minimum 42-45 HRC needed for adequate life
  • Resistance to fatigue crack initiation

Severe Loading

  • Core hardness of 46-50+ HRC needed
  • Maximum strength and fracture resistance

Section Size

  • Thicker sections require higher hardness to offset size effect
  • Account for hardness loss from low temperature tempering

The proper core hardness balances strength against fracture toughness. Design analysis provides guidance to avoid premature failure while minimizing brittleness.

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