Introduction
4140 steel is an alloy steel that provides an excellent combination of strength, toughness, and wear resistance. It is widely used for applications requiring high fatigue strength such as structural components, heavy equipment parts, axles, and aircraft landing gear.
To achieve these properties, 4140 steel plate must be heat treated using a process of quenching and tempering. Quenching rapidly cools the steel to form a very hard martensitic structure. Tempering then reduces the brittleness imparted by quenching to improve ductility and toughness.
This article provides an in-depth look at optimizing the quenching and tempering processes for 4140 steel plate to maximize strength and toughness. It covers critical parameters, recommended procedures, effects on microstructure, mechanical property response, and techniques tailored for heavy plate sections.
Overview of Quenching and Tempering
Quenching and tempering are heat treatments used to substantially increase the strength and hardness of steels like 4140 alloy steel by changing the microstructure.
The steps involved are:
- Heat to Austenitizing Temperature (>Ac3)
- Soak to Form Austenite
- Rapidly Quench to Form Martensite
- Temper at Controlled Temperature to Reduce Brittleness
- Air Cool to Final Condition
Proper control of the quenching severity and tempering conditions enables the optimal combination of strength and toughness needed for various applications of 4140 steel plate.
As-Rolled Microstructure
This banded structure and coarse grain size results in relatively low strength with tensile strength around 655 MPa (95 ksi). The ductility and notch toughness are also inadequate for critical components.
Quenching and tempering transforms this starting structure into much higher strength martensite to achieve the full capability of 4140 steel.
Formation of Austenite
The lamellar ferrite-cementite pearlite phases dissolve into a uniform face-centered cubic austenite structure. Alloy carbides also dissolve into the austenite.
The recommended austenitizing temperature range for 4140 steel is 1600-1650°F. The heavier plate sections require the higher temperatures to ensure complete dissolution and transformation.
Soaking at this temperature allows diffusion of carbon and alloying elements to form a homogeneous solid solution structure optimized for hardening. Typical soaking times range from 1-2 hours.
Quenching to Form Martensite
Following austenitization, 4140 steel plate must be quenched rapidly to transform the austenite into martensite. This diffussionless transformation produces a supersaturated tetragonal crystal structure with very high hardness and strength.
The recommended quenchant for 4140 steel plate is a hot oil bath maintained at 120-180°F. The oil provides rapid heat extraction while minimizing the risk of cracking from overly severe quenching.
Agitation of the oil bath improves heat transfer. Quench oils with high thermal conductivity, such as hot polymer types, maximize the hardening effect.
Cooling should be interrupted periodically to relieve residual stresses and avoid cracking of heavy plate sections. The load should be supported in multiple locations during quenching.
With proper 4140 steel plate quenching procedures, martensite hardness levels of 50-55 HRC are attained. However, the as-quenched structure is very brittle with low notch toughness. Tempering is necessary to improve these properties.
Tempering of Martensite
The quenched martensitic structure must be tempered to improve the ductility and toughness of 4140 steel plate while retaining high strength. This involves reheating to a controlled temperature below Ac1 and allowing partial decomposition of the martensite.
To obtain the optimal combination of strength and toughness, a double tempering treatment is recommended:
- Temper 1: Heat to 600-650°F, hold 1 hour, cool slowly
- Temper 2: Reheat to 1150-1200°F, hold 2 hours, air cool
The initial lower temperature temper reduces brittleness and allows transition carbides to form. The higher second temper maximizes tempered martensite formation for the best toughness.
For 4140 steel plate sections, slow cooling after each tempering step helps avoid untempered martensite on the surface. The tempering parameters may need adjustment for heavy sections to achieve uniform properties.
Tempered Martensite Structure
This structure provides an excellent combination of strength from the martensite matrix and toughness/ductility from the tempering process. The precipitated alloy carbides also enhance strength through precipitation hardening.
With proper tempering, the hardness of 4140 steel plate remains between 30-40 HRC for the optimum balance of properties desired in most applications.
Mechanical Properties
Quenching and tempering 4140 steel plate produces the following typical mechanical properties:
- Tensile Strength: 1450 MPa (210 ksi)
- Yield Strength: 1310 MPa (190 ksi)
- Elongation: 16% minimum
- Reduction of Area: 50% minimum
- Hardness: 33-38 HRC
Compared to the hot rolled condition, yield strength is approximately doubled while maintaining 15% elongation. This demonstrates the substantial strengthening effect of transforming the microstructure into quenched and tempered martensite.
The impact toughness of 4140 plate in the QT condition will generally exceed 15 ft-lbs at room temperature. Smooth or fine-grained surfaces enhance toughness.
Effect of Plate Thickness
As 4140 steel plate thickness increases, the hardness and strength produced by quenching and tempering may decrease, especially toward the plate center. This results from slower heat transfer causing lower martensite formation.
To compensate for lower hardenability in thicker sections, the following adjustments should be made:
- Increase austenitizing temperature to 1625-1650°F
- Lengthen soak times to 2 hours minimum
- Use maximum severity quenchant
- Utilize multiple tempering cycles
- Allow extra time for heat to equalize during quenching
With proper processing control, 4140 steel plate up to 8 inches thick can attain full properties. Thorough temperature monitoring is vital for heavy sections.
Tempering Effects on Toughness
While high strength is important, many applications for 4140 steel plate also require excellent toughness to resist impact and fatigue loading.
In general, lower tempering temperatures below 1000°F result in increased brittleness and reduced toughness due to high carbon martensite retention.
Conversely, tempering above 1150°F will over-temper the martensite and lower the strength of 4140 plate below the typical 180-210 ksi range.
Double tempering with the second temper from 1100-1150°F optimizes the toughness-strength balance. Testing should confirm over 15 ft-lbs Charpy V-notch impact toughness at room temperature.
Effect of Prior Microstructure
The properties obtained from quenching and tempering 4140 steel plate depend heavily on the prior microstructure before heat treatment:
- Coarse grained as-rolled plate has lowest response. Tensile strength 140-180 ksi range.
- Normalized plate has refined ferrite-pearlite grains. Tensile strength 160-210 ksi.
- Annealed and machined plate has uniform, stress-free structure. Tensile strength up to 210 ksi.
- Quenched and tempered plate has highest properties if re-austenitized. Tensile strength up to 230 ksi.
Maximizing the prior grain refinement, temperature uniformity, and freedom from residual stresses enhances the tensile strength and toughness achieved with 4140 steel plate quenching and tempering.
Effect of Quench Severity
The cooling rate during quenching significantly affects the final properties obtained in 4140 steel plate:
- Slow cooling in air only forms softened pearlite. Low 130-150 ksi strength range.
- Moderate rate oil quenching produces desired martensite structure. 190-210 ksi strength typical.
- Fast water quenching increases brittleness and risk of cracking. Best avoided for 4140 plate.
- Interrupted cooling minimizes distortion and cracking tendencies in plate. Allows equalization of temperatures.
Matching the quench severity to the plate thickness ensures optimal transformation to martensite while controlling distortion and cracking risks in 4140 steel.
Effect of Retained Austenite
Excessive retained austenite following quenching results in lower strength and toughness in 4140 steel plate. Adequate tempering should be performed to reduce retained austenite to less than 10% based on metallography.
Inadequate tempering or non-uniform cooling of heavy sections allows pockets of untempered austenite. This significantly decreases notch toughness and impact strength.
Optimal quenching and tempering is necessary to fully transform the 4140 steel plate microstructure into tempered martensite and minimize undesirable retained austenite.
Improving Fatigue Strength
Many applications, such as crane components and off-road vehicle frames, require 4140 steel plate to withstand high cycle fatigue loading.
Several measures can maximize the fatigue strength during heat treatment:
- Use lower end of tempering range: 650-750°F
- Maintain surface hardness below 38 HRC
- Produce fine surface finish with low residual stress
- Shot peening can induce compressive stresses
- Limit section sizes for faster quenching
With proper processing control, 4140 steel plate can achieve 110-120 ksi fatigue strength at one million cycles in a quenched and tempered condition.
Comparison to Carburizing
While quenching and tempering maximizes through-thickness properties, carburizing can produce even higher surface hardness up to 62 HRC in 4140 steel. However, depth of hardening is limited to 0.03-0.12 inches.
Carburizing is advantageous when extreme wear resistance or contact stresses require over 60 HRC at the surface. It can be applied to small 4140 components.
For most applications, quenching and tempering provides the best balance of surface hardness, case depth, strength, and fracture toughness needed in 4140 steel plate sections.
Example Applications
Here are some examples where 4140 steel plate is commonly quenched and tempered to obtain optimal mechanical properties:
- Crane and Derrick Structural Sections – Requires high yield strength combined with notch toughness. QT provides 210 ksi and 20 ft-lb Charpy.
- Loader Buckets – Must withstand high impact and wear resistance. QT achieves over 300 BHN Brinell hardness plus 15% elongation.
- Hydraulic Press Platens – Necessitates high compressive strength and damage tolerance. QT provides 190 ksi yield strength and 18 ft-lb impact.
- Off-Road Equipment Chassis – Needs fatigue strength above 100 ksi at 5 million cycles. QT enables 110 ksi fatigue strength.
- Pump Housings – QT achieves high tensile strength combined with ductility to resist pressure cycling.
With proper control of time, temperature, and quench severity, quenching and tempering consistently produces an optimal combination of strength, hardness, ductility, and toughness in 4140 steel plate for demanding applications.
Summary
For critical components made from 4140 steel plate:
- Heat uniformly to 1600-1650°F to completely austenitize
- Soak plate for 1-2 hours to homogenize
- Quench rapidly in hot agitated oil bath
- Double temper between 600-1200°F range
- Air cool to final condition
This quench and temper process transforms the microstructure into hardened martensite properly tempered for optimal strength >200 ksi and toughness >15 ft-lbs impact.
With close control of time, temperature, and quenchant parameters tailored to the plate thickness, an ideal balance of hardness, ductility, fracture toughness, and fatigue strength can be achieved with 4140 steel plate.
FAQ
Why quench and temper 4140 steel plate?
Quenching transforms the microstructure into very hard martensite while tempering reduces brittleness and improves toughness. This combination provides the high strength, hardness, and toughness needed in 4140 steel plate for demanding structural, mechanical, and aircraft applications.
What strength does quenching and tempering produce in 4140 plate?
With proper heat treating procedures, 4140 steel plate will attain a tensile strength over 200 ksi and yield strength above 180 ksi in a quenched and tempered condition. This represents over a 100% increase versus the hot rolled condition.
What tempering temperature should be used for 4140 steel plate?
To optimize toughness and ductility, 4140 plate should be double tempered – first from 600-650°F and then from 1100-1200°F. This properly decomposes the martensite for the best combination of strength and toughness.
How does plate thickness affect the quench hardening response?
Thicker plate sections over 2 inches cool slower during quenching, resulting in softer martensite formation toward the center. Maximizing austenitizing temperatures, soak times, and quench severity becomes more critical for achieving full properties through heavy sections.
Why is austenitizing temperature important for 4140 steel plate?
Higher austenitizing temperatures from 1600-1650°F ensure complete dissolution of alloy carbides and full transformation to austenite. This optimizes the quench hardening response for maximum martensite formation and hardness, even in the center of thick plate sections.
What special practices should be used when quenching thick 4140 steel plate?
Interrupted quenching, multiple tempering cycles, supporting the load properly, monitoring temperatures, allowing adequate equalization time, and slowing the cooling rate near the end help minimize distortion and cracking risks when quenching and tempering heavy 4140 steel plate sections.
How does quench rate affect the properties of 4140 steel plate?
Faster quench rates in hot oils maximize martensite formation for highest strength, but can increase brittleness. Interrupted quenching minimizes distortion. Air cooling only forms softened pearlite. Proper quench control ensures optimal properties.
What improves the fatigue strength of quenched and tempered 4140 plate?
Using the lower end of the tempering temperature range, restricting hardness to <38 HRC, inducing compressive stresses via shot peening, minimizing residual stresses, and maintaining fine surface finishes help maximize the fatigue strength of 4140 steel plate in a quenched and tempered condition.
What is the typical hardness range for quenched and tempered 4140 steel?
To obtain the optimum combination of strength, toughness and fatigue resistance, 4140 steel plate is normally quenched and tempered to a hardness range of 30-37 HRC on the Rockwell C scale. Higher hardness causes brittleness issues.
Can 4140 steel plate benormalized instead of annealed before hardening?
Yes, normalizing refines the grain size for improved hardenability and properties versus the as-rolled structure. The fine ferrite-pearlite structure enhances mechanical properties after quenching and tempering 4140 plate. Either annealed or normalized plate can be effectively hardened.