Introduction

The intense localized heating and subsequent cooling during welding results in steep thermal gradients in the 4140 steel weldment. This introduces significant residual stresses that can lead to cracking and warpage distortion if preventive measures are not taken.

Performing proper thermal stress relieving is vital for 4140 steel welds to remove these locked-in stresses and assure dimensional stability and service reliability. This article provides in-depth guidelines on selecting optimum post-weld stress relief parameters based on section thickness, inspecting for treatment effectiveness, and equipment best practices.

Following proven stress relieving protocols provides insurance against cracking and distortion in 4140 steel welded structures and components used in demanding applications across energy, transportation, construction, and other industries.

Need for Stress Relieving 4140 Steel Welds

During welding, intense localized heating causes adjacent base metal areas to thermally expand. Subsequent cooling contraction is constrained, resulting in residual tensile stresses sufficient to cause yield. Key consequences include:

• Transverse and longitudinal cracking in the weld or nearby base metal
• Distortion such as buckling, bending, or angular warpage
• Stress corrosion cracking when exposed to corrosive service conditions
• Premature fatigue failure under cyclic service loads
• Difficulty machining components due to warpage
• Inability to meet critical tolerance requirements

Stress relieving eliminates these detrimental effects by removing stresses created by welding prior to component final machining or entering service.

Overview of Stress Relieving Process

The objective of stress relieving is to uniformly heat the 4140 steel weldment to a suitable temperature below the critical phase transformation range, hold long enough for stresses to dissipate, and then slowly cool to hand-warm or room temperature. The process steps are:

• Clean welded component to remove dirt, oil, paint etc.
• Place part in furnace and heat to relieving temperature
• Soak at temperature for specified duration
• Cool slowly back to ambient temperature

This thermal exposure cycle relaxes peak residual stresses induced by welding. The key parameters of temperature and time must be selected appropriately.

Determining Optimal Stress Relief Temperature

Choosing the correct stress relief temperature is crucial. Key guidelines include:

• Minimum of 1200-1250°F to ensure sufficient thermal activation for stress reduction
• Below Ac1 temperature of 1333°F avoids altering microstructure
• Maximum of 1325°F to prevent overaging the heat affected zone
• Holding at optimal temperature enables stress dissipation

Insufficient temperatures fail to adequately relieve stresses. Excessive temperatures risks reduced strength, hardness, and crack resistance. Stress relief temperatures of 1200-1300°F are widely used for 4140 steel welds.

Recommended Stress Relief Times

Sufficient time at the relieving temperature must be allowed for stresses to decrease to acceptable levels. General guidelines are:

• Minimum 1 hour per inch of weldment thickness
• Allows thorough soaking at temperature
• Enables relaxation across entire cross section
• For plate weldments, soak 1 hour minimum
• Add 15 minutes for each additional inch over 1” thickness
• Double recommended time for structures with severe restraint
• Ensure adequate exposure for highly constrained joints

Prolonged exposure risks grain growth and over-tempering. The prescribed thermal cycles completely eliminate residual stresses while avoiding metallurgical changes.

Heating Methods for Stress Relieving

Various heating methods are utilized for stress relieving 4140 steel weldments:

• Electric resistance heating furnaces
• Fuel-fired furnaces using natural gas or propane
• Induction heating for localized areas
• Electric heating blankets on large fabrications
• Ovens with convection air circulation

The heating technique must allow maintaining a uniform target temperature across the entire component to ensure consistent stress reduction. Follow any manufacturer guidelines to avoid overheating or underheating areas during the process.

Verifying Stress Relief Effectiveness

Following stress relieving, the welded 4140 component should be inspected to verify effectiveness:

• Visually check for cracks using dye penetrant method
• Measure dimensional accuracy and confirm tolerances
• Compare hardness across weld, heat affected zone, and base metal
• Look for changes in weld appearance – oxidation, discoloration
• Monitor for rounded edges that indicate softening
• Perform strain gauge tests if required

This ensures the stress relieving cycle relaxed peak residual stresses as required. Adjustments to time or temperature can be made on subsequent weldments if needed.

Importance of Slow Cooling and Handling

To avoid reintroducing stresses after relieving, slow cooling and careful handling is critical:

• Slow furnace cool at 40-80°F per hour until hand-warm
• Cool wrapped in insulation for added protection
• Avoid drafts, fans, or chill bars during cooling
• Handle gently to prevent bending, flexing, or quenching

Slow cooling enables metallurgical changes initiated during stress relieving to fully take place. Careful handling prevents plastic deformation that could re-impose detrimental residual stresses.

Preventing Metal Loss and Oxidation

To avoid oxidation and loss of metal during prolonged stress relieving, it is advisable to:

• Use protective anti-scale coatings on parts prior to heat treatment
• Create a reducing atmosphere in furnace using dried hydrogen
• Limit the maximum air exposure at temperature
• Consider using vacuum furnace methods

These measures prevent scale formation, material loss, and carbon migration that could undermine the benefits of stress relief.

When to Stress Relieve During Manufacturing

Thermal stress relieving of 4140 steel weldments should be performed:

• After all welding operations are completed
• Following any cold working such as bending or straightening
• Before finish machining to final dimensional tolerances
• After secondary weld processes like surfacing or buttering
• Anytime severe residual stresses are suspected

The goal is to eliminate detrimental residual stresses at the most appropriate step just prior to final service exposure.

Proper stress relieving is an indispensable processing step for relieving residual stresses from welding on high-strength 4140 steel components. It minimizes risk of cracking and distortion that could result in catastrophic failures. The upfront investment in optimized stress relief practices pays major dividends in improved manufacturing quality, performance reliability, and safety assurance during end use applications.

Post Weld Stress Relief vs. Full Annealing of 4140 Steel

There are key differences between stress relieving and full annealing of welded 4140 steel fabrications:

Stress Relieving

• Temperatures of 1200-1300°F below phase change
• Only relieves weld residual stresses
• Microstructure largely unchanged

Full Annealing

• Temperatures of 1450-1550°F in austenite range
• Fully softens entire weldment
• Produces coarse ferrite-pearlite microstructure

Use of Stress Relieving

• After welding prior to finish machining
• Removes stresses while minimizing distortion

Use of Full Annealing

• When maximum ductility and toughness needed
• And loss of strength and hardness acceptable

Effects of Stress Relieving

• Dimensional stability for finish machining
• Reduced likelihood of cracking in service

Effects of Full Annealing

• Maximum weld zone ductility and fracture toughness
• Lowest hardness if joining dissimilar, hardened alloys

Stress relieving preserves base metal and weld properties while providing dimensional stability. Full annealing maximizes ductility and toughness at the expense of strength.

Effect of Stress Relief Temperature on 4140 Steel Weld Properties

The temperature used for stress relieving 4140 steel welds impacts the metallurgical changes and degree of stress reduction achieved:

Below 1200°F

• Only minor stress relaxation occurs
• Highest residual stresses remain
• HAZ microstructure unchanged

1200-1250°F

• Maximum residual stress removal
• No changes in base metal properties
• Preserves weld microstructure

1300-1350°F

• Some reduction in HAZ hardness
• Risk of slight overtempering at higher end
• Potential loss of strength if held too long

Above 1350°F

• Precipitation coarsening causes softening
• Strength and hardness reductions intensify
• Overaging embrittlement can occur

For 4140, holding at 1200-1250°F provides full stress relief while avoiding detrimental metallurgical changes that undermine properties. Temperatures must be precisely controlled.

Time Required for Effective Stress Relieving of 4140 Steel

Sufficient time at temperature must be allowed during stress relieving of welded 4140 steel to enable relaxation of residual stresses. Inadequate exposure can leave damaging stresses intact:

Plates Over 2” Thick

• Minimum 2 hours soaking time needed
• Add 30 minutes for each additional inch

Sections Under 2” Thick

• Minimum 1 hour exposure required
• Prevents under-exposure of thicker welds

Highly Restrained Joints

• May require double recommended time
• Ensures full stress reduction

Faster Heating Methods

• Can shorten required exposure times
• Due to accelerated stress relaxation effects

Joint Hardness

• Soften welds require shorter times
• Harder welds need prolonged exposure

Sufficient soaking provides insurance against inadequate stress relief. Exact times can be refined based on measurement of residual stresses using strain gauges.

Thermocouples Required for Monitoring Stress Relief Temperature

To accurately monitor temperatures during stress relieving of large 4140 steel weldments, an adequate number of thermocouples is required:

Workpiece Thermocouples

• Attach thermocouples directly to weldment
• Indicates actual part temperature
• Identifies any cold spots or hot spots

Grid Pattern

• Space thermocouples evenly throughout component
• Measures temperature uniformity

High and Low Points

• Thermocouples on thickest and thinnest sections
• Ensures proper soaking of entire piece

Furnace Thermocouples

• Additional thermocouples placed inside furnace
• Monitors furnace hot zone temperature

Data Logging

• Automated temperature data collection
• Provides record of thermal cycle

Monitoring with sufficient thermocouples verifies all areas of the weldment reach the target stress relief temperature for the required duration to completely remove residual stresses from welding.

Effects of Improper Cooling After Stress Relieving 4140 Steel

Slow, controlled cooling is essential after stress relieving to avoid reintroducing detrimental residual stresses. Consequences of excessively fast cooling include:

Rehardening and Embrittlement

• Rapid cooling rehardens previously stress relieved areas
• Causes renewed susceptibility to embrittlement

Renewed Distortion

-thermal gradients from quick cooling restore residual stresses

• Leads to recurring warpage or buckling

Cracking

• Delayed hydrogen cracking enabled by rehardened zones
• Stress corrosion cracking in presence of corrodents

Loss of Dimensional Tolerance

• Distortion and warpage impede holding tight machining tolerances
• Affects fit-up and performance of assemblies

Re-machining

• Distortion may necessitate re-machining of finished surfaces
• Causes manufacturing delays and cost overruns

Care must be taken to properly cool 4140 steel weldments after stress relieving. Slow cooling in furnace or insulation prevents undoing the benefits of the heat treatment.

Effect of Preheating on Residual Stresses in 4140 Steel Welds

Preheating weld joints helps minimize residual stresses. Benefits include:

• Expands the steel, putting joint in tension
• Offsets contraction stresses during cooling
• Reduces thermal gradients between weld and base metal
• Lessens peak tensile stress during cooling
• Slows the cooling rate through transformation range
• Allows more time for stresses to dissipate
• Improves hydrogen diffusion and degassing
• Lowers risk of hydrogen cracking
• Makes the joint more ductile and less prone to cracking
• Enables use of slower weld cooling without cracking

For 4140 steel, preheating to 200-300°F optimizes properties and reduces residual stresses and the corresponding risk of cracking. But full stress relief is still required after welding is completed.

Methods For Accelerated Stress Relief of 4140 Steel Weldments

In some cases accelerated stress relief techniques offer advantages over traditional slow furnace heating:

Induction Heating

• Alternating electromagnetic field rapidly heats part
• Speeds reaching required temperature

Resistance Heating

• Running electrical current through part quickly increases temperature
• Minimizes heating equipment and space

Flame Heating

• Oxy-fuel torches heat localized areas rapidly
• Useful for small parts or pre/post heating

Dielectric Heating

• RF energy instantly generates heat in poor conductors
• Rapidly penetrates cross section

Infrared Heating

• IR modules produce fast thermal response
• Ideal for heating near surfaces and edges

Vacuum Stress Relief

• Vacuum furnace accelerates heat transfer
• Minimizes oxidative loss of properties

Faster heating activates quicker stress reduction kinetics. This facilitates shorter exposure times to relieve stresses. The techniques must be carefully applied to avoid overheating.

Preventing Reheat Cracking During Stress Relief of 4140 Steel

Reheat cracking is intergranular fracture caused by formation of grain boundary carbides during cooling after stress relieving. To avoid:

• Limit maximum temperature to 1300°F
• Lower temperature reduces carbide precipitation
• Closely control cooling rate through critical range
• Cool quickly through 1300-1000°F carbide formation range
• Lower carbon content lessens carbide potential
• Chromium and molybdenum also promote carbides
• Stress relieve at lowest adequate temperature
• Higher exposure temperature accelerates cracking
• Pre or post heating to 1000°F allows carbides to form before stress relief
• Reduce grain boundary weakness by microalloying
• Elements like boron and titanium stabilize boundaries
• Avoid overheating or excessive dwell times
• Accelerates grain boundary carbide formation

Close control of temperature, time, cooling rate, and chemistry is required to sidestep reheat cracking during stress relief treatment of 4140 steel fabrications.

Using Portable Induction Heaters for Localized Stress Relief

Portable induction heating equipment allows targeted stress relief of small areas and edges prone to cracking:

• Concentrates heat in specific weld zones
• Avoids completely reheating large components
• Permits selective stress relief after part is machined
• Reduces distortion compared to full furnace treatment
• Localized nature avoids metallurgical changes
• No risk of overtempering unaffected areas
• Facilitates tight temperature control
• Faster heating and cooling cycles
• Small portable units can be moved to welds
• Powered from standard wall outlets
• Used for crack repairs and construction retrofits
• Stress relieves repaired welds or added welds

Portable induction enables cost-effective localized stress relief on 4140 steel weldments. The precision and speed minimizes distortion while maximizing residual stress removal.

Magnetic Particle Inspection of 4140 Steel After Stress Relief

Magnetic particle testing after stress relieving 4140 steel welds validates crack removal and detects any new flaws:

Principles

• Magnets magnetize part to enhance surface defects
• Ferromagnetic powder clusters at flux disturbances

Crack Detection

• Small surface and subsurface cracks revealed
• Verifies cracks healed by stress relieving

New Discontinuities

• Detects any new cracks from stress relief
• Shows boundaries of repaired weld zones

Simplicity

• Quick, easy method needing minimal equipment
• Highly portable for field inspections

Limitations

• Unable to detect cracks in non-ferrous alloys
• Penetrates only shallow subsurface areas

Magnetic testing provides inexpensive proof stress relieving eliminated cracking while monitoring for any new defects. It offers quick go/no-go evaluation of 4140 steel weldments.

Benefits of Automated Temperature Recording During Stress Relief

Using chart recorders or data loggers to monitor furnace temperature provides documentation and quality assurance:

• Plots entire thermal cycle signature
• Verifies temperature uniformity
• Confirms metal temperature not furnace air temp
• Validates prescribed temperature was achieved
• Records time at temperature
• Provides proof proper soaking duration reached
• Logs cooling rate
• Checks for excessively rapid cooling
• Provides historical record
• Enables analysis of heating and cooling behavior
• Aids optimizing future treatments
• Supports statistical process control
• Alerts to any temperature control issues
• Signals equipment faults or calibration needs

Data logging creates objective evidence of process compliance. It is an essential productivity tool for quality, analysis, and efficiency in stress relief of 4140 steel.

Nondestructive Testing of Stress Relieved 4140 Steel Weldments

Several nondestructive examination methods can evaluate 4140 welds after stress relief without harming the component:

Visual Inspection

• Checks for surface irregularities indicating flaws
• Verifies acceptable weld shape and appearance

Liquid Penetrant Testing

• Detects surface breaking defects like cracks
• Indications appear after developer is applied

Magnetic Particle Testing

• Detects surface and near-surface flaws
• Color contrast improves visibility

Ultrasonic Testing

• Measures internal discontinuities and thickness
• Maps sub-surface defects requiring repair

Radiographic Testing

• X-ray or gamma rays reveal internal voids and porosity
• Shows weld fusion and core condition

Multiple NDT methods provide comprehensive proof of sound welds, free of detrimental defects and protected from failure by proper stress relieving. They confirm integrity without damaging parts.