Understanding Hardenability of Steel: Definition, Measurement, and Practical Applications

When discussing quenching—a specific type of steel heat treatment—many people hold a common misconception: that "hardenability" determines the total amount of hardness gained after quenching. In reality, the hardenability of steel does not dictate the degree of hardness. Instead, it determines whether the hardened portion of a steel part remains only on the surface or penetrates deep into the core.

 

The choice between low and high hardenability determines whether your CNC machined steel parts achieve a "hard shell with a tough core" or a "solid, uniform consistency." Misidentifying the hardenability of a steel grade can lead to critical failures—either the core lacks sufficient strength, leading to snaps, or the entire part becomes too brittle and cracks, resulting in wasted budget and mechanical failure. This article will explore what is hardenability in depth and explain how the Jominy hardenability test measures this property to help you select the best steel grades for various CNC machining applications.

 

 

 

 

 

What is Hardenability?

 

 

Simply put, hardenability measures the "depth" to which an alloy steel can be hardened from the surface to the interior during the quenching process. It is vital to distinguish a core concept: hardenability is not necessarily linked to "how hard" a steel can get. ‘How hard” a steel can get is achieved depends primarily on the steel’s chemical structure and carbon content. 

 

For instance, within the 440-series martensitic stainless steels that share the same microstructure, grades with higher carbon content will achieve a higher peak hardness after quenching. In contrast, hardenability determines the distribution of that hardness across the part's cross-section—whether only the surface is hardened (low hardenability) or the entire thick cross-section is hardened (high hardenability).

 

 

 

 

 

This difference in hardenability directly affects the service life and application of the part. For example, heavy-duty gears require low hardenability to achieve a "wear-resistant surface and shock-resistant core." Conversely, structural pins used in construction require high hardenability to ensure overall strength; they will not fracture unless the external force exceeds their ultimate strength.

 

 

 

 

It is worth noting that while steel with high hardenability can theoretically undergo "shallow quenching" (by controlling temperature and time), it is practically difficult to prevent the interior from hardening. On the other hand, steel with low hardenability simply cannot achieve through-hardening in thick cross-sections, regardless of the process.

 

So, besides 440C martensitic stainless steel, what other types and grades of steel can be hardened by quenching? And what is their specific hardenability?

 

 

 

 

Steel Types and Grades: Low vs. High Hardenability  

 

 

Steels that can be quenched and hardened are primarily carbon steels, martensitic stainless steel, tool steel, and alloy steel (spring steel, bearing steel).

 

Below, we categorize common quench-hardenable steels—from high-carbon steels like 1095 to advanced alloy tool steels like H13—to show how they differ in their hardening depth.

 

 

 

 

 

Key Observations:

 

• Hardenability of 1018 Steel: As seen in the table, the hardenability of 1018 steel is exceptionally low. Even after quenching, the core remains soft (under 20 HRC). This makes it ideal for parts needing a "tough heart" but unsuitable for structural components requiring uniform strength.
  
• Hardenability of 4140 Steel: In contrast, the hardenability of 4140 steel is much higher due to its Chromium and Molybdenum content. You will notice the hardness drop from the surface to the core is minimal (only 4-5 HRC difference), ensuring high tensile strength throughout the entire cross-section.
  
• Tool Steels (D2 & H13): These are designed for molds and dies, they achieve nearly identical hardness from the outside in, which prevents the part from collapsing or deforming under massive industrial pressure.

 

 

 

 

The Jominy End Quench Hardenability Test

 

 

The Jominy End Quench test is used to evaluate the hardenability of steel. The standard operating procedure is as follows:

 

• Specimen Preparation: First, a standard Jominy specimen is prepared. The specimen is typically a cylindrical bar with a diameter of approximately 25.4 mm (1 inch) and a length of 102 mm (4 inches). One end of the specimen is machined with a flange for mounting.
  
• Austenitizing: The Jominy specimen is heated to its austenitizing temperature and held for a sufficient duration to ensure the internal structure is completely transformed into austenite. This step is critical for ensuring accurate quenching results.
  
• End Quenching: The heated specimen is placed vertically in a specialized fixture. A standard nozzle sprays water at a constant flow rate and pressure onto the bottom (end) of the specimen. The rest of the specimen cools in the air. This creates a continuous cooling rate gradient from the quenched end to the far end.
  
• Hardness Measurement: Once cooling is complete, a thin layer of material is ground off one or two sides (usually opposite sides) to create parallel flat surfaces. Rockwell hardness measurements are then taken along these flats at specific intervals (e.g., every 1.5 mm or 1/16 inch) starting from the quenched end.
• Plotting the Hardenability Curve: The measured hardness values are plotted against their corresponding distance from the quenched end to generate the steel's hardenability curve.

 

Here I show a typical Jominy curve for the : “Case Hardening” of the 1018 steel and “Through Hardening” of the 4140 steel:

 

 

 

 

(The blue curve (1018) exhibits a faster hardness decline and lower hardness values at greater distances from the quenching end, indicating poorer hardenability. The red curve (4140) maintains higher hardness levels with a slower rate of decline, suggesting superior hardenability.)

 

 

 

 

CNC Machining Applications: Case Hardening vs. Through Hardening

 

 

For steels being CNC machined into precision parts, selecting the appropriate hardening strategy based on the part's requirements and the steel's hardenability is necessary:

 

 

Low Hardenability (Case Hardening)

 

When a steel has low hardenability, case hardening is typically used to harden only the surface layer while the core remains relatively soft and tough.

 

• Industries: Automotive transmission, heavy machinery, agricultural equipment.
  
• Parts: CNC machined heavy-duty gears, camshafts, pins, and crankshafts.
  
• Rationale: These parts require extremely high surface hardness and wear resistance to withstand friction, fatigue, and contact stress. Simultaneously, the core must maintain excellent toughness to absorb impact loads and torque, preventing brittle failure. The case provides wear resistance while the tough core provides impact resistance and load-bearing capacity.

 

 

High Hardenability (Through Hardening)

 

When steel possesses high hardenability, through-hardening can be performed to achieve high, uniform hardness throughout the entire cross-section of the part.

 

• Industries: Construction, aerospace, tool manufacturing, mold and die industry.
  
• Parts: High-strength fasteners (bolts, nuts), drill bits, milling cutters, punches, structural load-bearing pins, and molds.
  
• Rationale: These components generally require high overall strength, hardness, and shear resistance to endure high stress, heavy loads, and abrasive wear. For example, cutting tools need through-hardness to maintain sharp edges, and fasteners need overall strength to ensure connection reliability.

 

 

 

 

Successful Project in VMT CNC Machining Factory

 

 

At VMT CNC Machining Factory, we are committed to providing customers with one-stop solutions—from material selection to machining and heat treatment—ensuring that the final product meets rigorous performance standards.

 

• Client Need: A construction machinery manufacturer required a critical pivot pin for a heavy-duty excavator arm connection. This pin must withstand massive radial and axial loads, requiring extreme surface wear resistance against frequent friction and corrosion, while the core must possess outstanding impact toughness to prevent sudden fracture under large impact.
  
• Our Suggestion: Based on a deep analysis of the operating environment and performance requirements, as well as a professional understanding of material hardenability, we recommended 4140 alloy steel instead of the client's originally planned 1045 carbon steel. 4140 offers better hardenability. It allows for better control of the hardening depth and ensures the core maintains its necessary toughness during the surface hardening process.
  

 

Process:

 

• CNC Machining: We utilized advanced 5-axis CNC machining centers to perform high-precision turning, milling, and drilling on 4140 steel bar stock. Complex geometries were completed in a single setup to ensure dimensional accuracy and surface finish requirements.
  
• Targeted Heat Treatment (Quenching): Post-machining, the pins underwent a strict heat treatment process:
  
• Austenitizing: Heating to the 4140-specific austenitizing temperature with precise soak time.
  
• Oil Quenching: Using oil quenching to achieve a martensitic structure at a moderate cooling rate, which is vital for deep hardening in 4140 while avoiding the cracking risks associated with water quenching.
  
• Low-Temperature Tempering: Following quenching, the pins were tempered to relieve internal stresses, enhance toughness, and reach the target hardness range.
  
• Result: Through this series of precision CNC machining and targeted heat treatments, the final 4140 alloy steel pins achieved the perfect hardness distribution: a surface hardness of 58–62 HRC for excellent wear resistance, and a core maintained at 30–35 HRC for superior impact toughness. The client was highly satisfied with the "one-stop" solution and the exceptional performance of the product, establishing VMT as a long-term partner.

 

 

 

 

FAQs

 

 

What is the difference between hardness and hardenability?

 

Hardenability determines the extent to which a steel part becomes hard—whether it hardens only on the surface (low hardenability) or throughout the entire component from the inside out (high hardenability). The alloying elements within the steel dictate the level of its hardenability.

 

Hardness is typically measured using the Rockwell (HRC) or Brinell (HB) scales. Higher values indicate better wear resistance. For steels that can be quench-hardened, the resulting hardness is primarily determined by carbon content; higher carbon levels generally result in greater hardness.

 

 

 

Why is hardenability important?

 

Hardenability determines whether:

 

• You need parts with a "Hard Shell and Tough Core" (high wear resistance on the surface with a ductile, impact-resistant interior) — such as: impact-resistant heavy-duty gears, Camshafts and Crankshafts, Self-tapping Screws, and Landing Gear components;
  
• Or you need parts that are "Hard Throughout" (uniformly hardened from the surface to the center) — such as: high-strength construction bolts, heavy-duty tie rods and load-bearing pins, Drill Bits and Milling Cutters.
  

 

The former can be achieved through low hardenability, while the latter can be achieved through high hardenability.

 

 

How is hardenability measured?

 

Hardenability is most commonly measured using the Jominy End-Quench Test. In this process, a standardized steel specimen is heated to its austenitizing temperature and then quenched at one end with a controlled water spray. Hardness is then measured at specific intervals along the length of the bar to create a Jominy curve, which shows how deeply the steel can harden as the cooling rate decreases.

 

 

How to increase the hardenability of steel?

 

Hardenability is increased by adding alloying elements such as Chromium, Manganese, Molybdenum, and Nickel. It is more common to add small amounts of multiple elements together to achieve a synergistic effect, as seen in 4340 alloy steel, which contains a combination of Ni, Cr, and Mo to ensure deep hardening.

 

 

What factors affect hardenability?

 

Several factors determine a steel's hardenability. Chemical composition and alloying elements are the primary factors. The steel’s microstructure, specifically the austenite grain size, also plays a role, as larger grains generally increase hardenability. Additionally, the part size and shape affect heat dissipation, and the choice of quenching media, such as water, oil, or brine, determines the actual cooling rate achieved.

 

 

Do all steels have the same hardenability?

 

No, steels vary significantly in their hardenability based on their composition. Plain carbon steels like 1018 have very low hardenability and only harden on the surface. In contrast, alloy steels like 4140 or 4340 have high hardenability, allowing them to be hardened through their entire cross-section even in large diameters.