The Comprehensive Guide to Flame Hardening: Benefits and Technical Insights

Flame hardening is a distinguished method in the domain of heat treatments for steel components, including gears. The technique uses an oxy-fuel flame to selectively heat parts to their austenitizing temperature, followed by a quenching process.

Process Overview: Flame Hardening 4140 & Other Steel Grades

  • Depending on the alloy and degree of machining, a preheat operation may be initiated.
  • The component is fixtured to a spin table, and torch heads are adjusted for optimal positioning.
  • Ignition of the torches heats the part surface to the desired temperature, followed by an automatic quench in an agitated polymer solution.
  • The part is immediately transferred to tempering, reaching the intended hardness range.
  • The outcome is a hardened case layer on the material’s heat affected zone, while the core remains largely ductile.

Technical Benefits

  • Localized Treatment: Only specific areas, such as gear teeth, can be hardened without altering the entire component’s properties.
  • Cost-Effectiveness: Flame hardening is economical due to its less complex equipment needs.
  • Surface Hardness: Offers a hard exterior with a ductile core, optimizing wear resistance and shock absorption.
  • Speed & Efficiency: The technique is swift and suitable for large-scale production.
  • Minimized Distortion: Only heating the wear surface instead of the entire part reduces the risk of warping.
  • Versatility: Compatible with various steel alloys.

ThermTech’s Capabilities in Flame Hardening

  • Advanced Equipment: ThermTech operates a fully automated flame table with infrared temperature control, ensuring uniform and precise heating.
  • Custom-Design Tooling: With a potential 2-4 week addition depending on design intricacy, ThermTech can craft tooling for unique geometries.
  • Flexibility: ThermTech can adjust the polymer quench system concentration to match different alloy grades and configurations.

Recommendations & Specifications

ThermTech recommends spin flame hardening for parts weighing up to 2,500 pounds and with a maximum OD of 45”. While a multitude of materials can be treated, commonly processed ones include:

  • 4140 Steel
  • 4150 Steel
  • 1045 Steel
  • 1060 Steel
  • Ductile Cast Iron (DCI)

The method is especially suitable for:

  • Large gears or components where conventional quenching is impractical.
  • Cases with strict dimensional tolerances.
  • Parts needing hardness on specific segments or surfaces.

For gears, the common alloys processed include 1045, 1060, 4140, and 4340 gears.

It is paramount for clients to go through the formal quoting process to ensure the highest quality outcomes.


Flame hardening offers a synergy of economic and technical benefits. With expertise like ThermTech’s, steel components and gears can gain improved wear resistance, longevity, and performance. This process stands as a testament to the evolution of heat treatment, enhancing the properties of steel components and gears to meet the challenges of modern engineering and application demands.

ThermTech—Experts in Austempering Stampings, Machined Parts, and Investment Castings

ThermTech, Inc. offers both small and large batch capabilities for austempering steel and iron investment castings. Between our neutral salt bath furnaces and our larger austempering furnaces, we have the capabilities to austemper a wide variety of stamping, machined components, and investment castings. We have run austempering on alloys from 1060 through 4340 to ductile iron machined parts.

Our dedicated team works with you to ensure we meet your specifications. Count on our decades of experience with hardening, tempering, surface heat treatments, vacuum treatments, and annealing. To learn more about our services, please contact us today.

Learn more about austempering and how ThermTech can serve you by watching these videos.

Request a free quote online or call us at 800-752-6917. You can also contact us online for all your austempering and heat treating needs.


    ThermTech’s quality policy is ZERO DEFECTS for all customer products and in all quality management system activities. Through our efforts to continually improve the quality of the work we offer, we’ve been recognized as being able to meet the rigorous quality standards required by several of our major customers. ThermTech is certified for ISO 9001 and AS9100 in both Metal Heat Treat and Metal Finishing.