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Through Hardening

Rapid quenching can produce stronger steel, throughout your critical parts.

Increased strength, inside and out.

Through hardening, also known as neutral hardening, is the process of strengthening a steel alloy using a rapid quench for increased hardness throughout the material. This process matches the carbon throughout the entire section of steel, as opposed to case hardening, which creates a hard outer layer while preserving a softer, more ductile core.

Through hardening applications

Through hardening is useful for applications where strength, hardness and wear-resistance are critical. It’s a process that’s used in a wide range of engineering applications, including:

  • Seat frames and seat belt buckles
  • Hand tools
  • Parts that will need to sustain heavy loads during their service life, such nuts and bolts, brackets, chains, nails and hooks
  • Springs, axles, bearings, blades, scrapers and other miscellaneous components used in industry

Through hardening vs tempered steel

Through hardened steel is sometimes referred to as “tempered steel,” but there’s a difference. Tempering is simply the process of reheating a material following the initial heating and quenching to combat brittleness. While it’s conducted in many heat treating processes, it does not necessarily mean that an alloy has been through hardened.

Through hardening typically consists of three steps:

  • Heating an alloy to alter its physical properties.
  • Rapid quench in a medium, such as oil, salt or caustic.
  • Reheating, or tempering, to eliminate excess brittleness from the treated alloy.

High tech solutions for heavy duty jobs.

All through hardening jobs are monitored, from order placement to delivery, by our state-of-the-art Production Information and Customer Service system (PICS to guarantee technical and engineering precision for all of our through hardening projects, from quality control to on-time delivery, and everything in-between.

We have oil quench furnaces capable of handling cycles up to 3,500 pounds. We can handle parts measuring up to 48 inches by 36 inches by 36 inches in batch processing and continuous processing of up to 4,000 pounds per hour.

Paulo delivers:

  • Collaborative scheduling solutions to ensure on-time delivery
  • Large variety of furnace and process options
  • Superior process control for in-range hardness results every time
  • Custom-designed washing equipment to clean parts before and after processing

Start a conversation.

Get stronger parts with our through hardening services. Request a quote or contact us today to learn about the Paulo difference.

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Through Hardening FAQs

Can Paulo through harden to aerospace (AMS) specifications? +

Yes, our Kansas City and Cleveland facilities offer Nadcap-accredited vacuum hardening that meets AMS specifications. We also offer oil quenching in Kansas City. For commercial through hardening without aerospace requirements, we provide atmosphere and vacuum hardening capabilities.

What is through hardening? +

Through hardening is a heat treatment process that increases hardness and strength throughout the entire cross-section of a steel part. Parts are heated above their critical temperature (typically 1500-1600°F), then rapidly quenched in oil, polymer, or gas to form a hard martensitic structure. Unlike surface treatments, through hardening creates uniform strength properties from surface to core, making it ideal for components requiring structural integrity.

What other names are used for through hardening? +

Through hardening is also called neutral hardening, particularly in atmosphere furnaces where carbon-neutral conditions prevent surface changes. When performed in vacuum furnaces, it’s often called vacuum hardening. All three terms describe the same fundamental process of hardening the entire part cross-section.

What's the difference between through hardening and case hardening? +

Through hardening creates uniform hardness throughout the entire part for structural strength—think seat belt buckles or bridge bolts that need uniform strength. Case hardening creates a hard surface layer (the “case”) while maintaining a softer, ductile core—ideal for gears and shafts that need wear resistance outside but toughness inside. Through hardening is best when parts need overall strength; whereas case hardening for wear resistance.

What's the difference between through hardening and nitriding? +

Nitriding is actually a type of case hardening that adds nitrogen to create a thin, hard surface layer for wear and corrosion resistance. Through hardening transforms the entire part’s microstructure for overall strength. Think of nitriding as armor plating for the surface, while through hardening is strengthening the entire structure.

Which materials can be through hardened? +

Through hardening requires sufficient carbon and alloying elements. Common materials include: Engineering alloys: 4140, 4130, 4340 Tool steels: A2, D2, S7, H13 Stainless steels: 410, 420, 440C You cannot through harden: Low-carbon steels like 1018, 1010, or 1008—these require case hardening for surface hardness.

Will my entire part be the same hardness throughout? +

Not necessarily. While thin parts achieve uniform hardness, thicker sections often show decreasing hardness toward the center. This “mass effect” occurs because the core cools slower than the surface during quenching. A 4140 shaft might achieve 50 HRC at the surface but only 35 HRC at the center if it’s over 3 inches thick. We test surface hardness unless core hardness verification is specifically requested.
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