Why Knives Crack in Quench: Distortion, Residual Stress, and Maker Controls That Prevent It

By Yashar Mousavand – Lead Instructor

What “quench cracking” actually is

Quench cracking is a hardening failure where a blade develops one or more cracks during or shortly after the quench. The root cause is internal stress. Quenching creates steep temperature gradients and triggers the austenite-to-martensite transformation. Those two effects generate thermal stress and transformation stress. If the local tensile stress exceeds the steel’s strength or fracture resistance at that moment, a crack initiates and propagates. [2][3]

Distortion vs residual stress vs cracking

Distortion and cracking are related outcomes of the same stress field. Distortion is permanent shape change caused when quench-generated stresses drive plastic strain. Residual stress is what remains locked-in after cooling. Cracking occurs when tensile stress concentrates enough to open an existing defect or a geometric stress riser, or when microstructural heterogeneity creates weak local zones. [2][3][8]

The two stress engines in a quench

1) Thermal stress from nonuniform cooling

Nonuniform heat transfer means different parts of the blade contract at different times. The resulting restraint creates tensile and compressive stresses. During immersion quenching, heat transfer progresses through distinct regimes. Early film boiling can create a vapor blanket that insulates the surface. When that blanket collapses, nucleate boiling provides much higher heat transfer, followed by convection at lower temperatures. If different regions of the blade transition between regimes at different times, cooling rate becomes highly nonuniform, amplifying thermal gradients, distortion, and crack risk. [2][4][5]

2) Transformation stress from martensite formation

As the blade cools through its martensite start range, austenite transforms into martensite. This transformation involves a volume increase and introduces additional stress. If martensite forms locally, the local volume change is also local, creating stress concentrations. Transformation induced plasticity (TRIP) further couples stress and transformation, meaning even stresses below yield can produce permanent strain during the phase change. [1][2]

Why blades are especially crack-prone

Knife geometry concentrates stress. A thin edge sits next to a thicker spine. Plunge lines, tang shoulders, and holes for pins or lanyards create abrupt section changes. Sharp corners cool faster and concentrate stress. Surface defects and deep scratches act as crack starters. Overheating during austenitizing can coarsen grains and reduce crack resistance. [3]

Where cracks usually start

• Geometric stress risers: sharp inside corners, steep edges around holes, grooves, and abrupt section changes. [3]
• Surface or subsurface defects: inclusions, laps, seams, and damage introduced during machining or grinding. [3]
• Regions of locally higher cooling rate: sharp edges and corners that enter high heat transfer regimes first, or areas with stronger quenchant flow. [4][5]
• Areas with microstructural heterogeneity: mixed transformation products or uneven austenitizing leading to local brittleness. [8]

Maker controls that prevent cracking

Control 1: Start the quench with low internal stress

If a blade already contains machining and grinding stress, quench cracking risk increases. Stress relieving after rough machining reduces embedded stress so it does not add to quench stress. Uddeholm describes stress relieving tool steels after rough machining at 550 to 700 C, holding 2 to 3 hours once temperature is uniform, then slow cooling (for example in the furnace) to avoid creating new thermal stress. [1]

Control 2: Heat evenly before you cool fast

Rapid or uneven heating increases thermal stress and can promote cracking. Stepped heating with preheats helps equalize temperature between surface and core before final austenitizing. Uddeholm recommends slow ramping and stepped preheats, commonly around 600 to 650 C and 800 to 850 C, to reduce temperature differences between surface and center. [1]

Control 3: Choose quench severity to match the steel and the geometry

More severe quenching usually increases residual stress. Published studies summarized in a quench hardening review show water quenching produces the highest residual stress magnitude compared with less severe media, and that increasing polymer concentration can decrease residual stresses by reducing quench severity. Oil quenching also shows lower residual stresses than water in comparative studies. [2]

Control 4: Make heat extraction uniform

Uniformity beats speed. Uddeholm notes that very powerful stresses arise during quenching, and that excessively rapid and uneven quenching can cause local martensite formation and local volume increases that generate stresses, leading to distortion and in some cases hardening cracks. They also emphasize applying the quench medium as uniformly as possible to avoid temperature differences that cause distortion. [1]

Practical controls include: stable bath temperature, repeatable agitation, avoiding part-to-part shielding, and consistent part orientation. Heat Treat Doctor guidance also highlights that agitation, improper fixturing, and crowding or orientation can contribute to quench cracking. [3]

Control 5: Use interrupted quenching when geometry demands it

Interrupted quenching processes such as martempering (also called marquenching) reduce thermal gradients by quenching into a hot medium above the martensitic range, holding until the temperature throughout the piece is substantially uniform, then cooling at a moderate rate through the transformation range, followed by tempering. Commercial heat treaters describe this specifically as a method to minimize distortion, cracking, and residual stress. [6]

Control 6: Temper immediately

After quench, martensitic structures contain high internal stress. Uddeholm notes that the structure after hardening contains inherent stresses that can cause cracking, and that tempering lowers these stresses, so tempering should follow immediately after hardening. Heat Treat Doctor likewise stresses that tempering is important to relieve stress and should be done without delay to reduce cracking risk. [1][3]

A practical quench-crack prevention checklist for blades

Design and prep

• Blend transitions and remove sharp internal corners where possible. [3]
• Keep edge thickness consistent before heat treat to reduce thermal gradients.
• Eliminate deep scratches at stress-critical zones (plunge, ricasso, tang shoulders, holes).
• Stress relieve after rough grinding or rough machining when geometry is complex. [1][3]

Austenitize with discipline

• Use stepped heating and preheats to equalize temperature before final austenitizing. [1]
• Avoid temperature overshoot and long soaks that promote grain growth and reduced crack resistance. [3]
• Use thermocouples or a proven furnace process to confirm the core reaches temperature without overheating the surface. [1]

Quench execution

• Select a quenchant with appropriate severity. Expect higher stress and higher crack risk with very severe quenchants like water. [2]
• Control bath temperature and agitation. Aim for repeatable, uniform heat extraction. [2][3]
• Avoid part-to-part shielding. Quench one blade at a time if needed. [3]
• Be consistent in insertion orientation and motion. Random motion creates random cooling.
• If you have access to martempering or marquenching, consider it for crack-prone geometries. [6]

Post-quench

• Temper immediately after the blade reaches safe handling temperature. [1][3]
• Use multiple tempers when the steel family and application call for it, and follow the steel maker’s datasheet. [1]
• If you use sub-zero treatments, treat them as part of a documented sequence, not a last-minute fix.

How to diagnose a crack so you can fix the process

Treat every crack as a data point. Document steel, blade geometry (including thickness map), austenitizing temperature and time, quenchant type and temperature, agitation method, orientation, time to temper, and whether the blade was stress relieved. Then look for patterns: cracks at holes and shoulders often implicate stress risers; random cracks across batches often implicate inconsistent quench conditions; delayed cracks often implicate temper delay and high residual stress. [2][3]

Why Knives Crack in Quench: Distortion, Residual Stress, and Maker Controls That Prevent It

What causes quench cracking?

Quench cracking happens when internal tensile stress during or right after quenching exceeds the steel’s instantaneous fracture resistance. The stress comes from two coupled drivers: thermal stress from nonuniform cooling (different regions contracting at different times) and transformation stress as austenite turns into martensite with a volume increase. Cracks usually initiate at stress concentrators such as sharp transitions, holes, plunge lines, tang shoulders, or at defects and locally brittle microstructures created by overheating or uneven austenitizing.

How to prevent quench cracking?

You prevent quench cracks by lowering peak tensile stress and removing places where that stress concentrates.

• Reduce stress risers: add radii, avoid sharp internal corners, keep section changes smoother, and eliminate deep scratches at shoulders and plunge lines.
• Start with low internal stress: stress relieve after rough grinding or machining so pre-existing stress does not add to quench stress.
• Heat evenly: use stepped preheats and controlled ramping so the blade enters the quench with minimal temperature gradients.
• Match quenchant severity to steel and geometry: overly severe quenching increases residual stress and crack risk.
• Make heat extraction uniform: control bath temperature, agitation, orientation, and avoid part-to-part shielding.
• Use interrupted quenching when needed: martempering reduces thermal gradients and crack risk for crack-prone geometries.
• Temper immediately: as-quenched martensite contains high inherent stress, and delaying tempering increases cracking risk.

Is it better to quench with water or oil?

Neither is universally “better.” It depends on steel hardenability, cross-section, and the hardness you need. Water is a much more severe quenchant and is associated with higher residual stresses and higher crack risk; reviews and your article both point out water quenching produces the highest residual stress magnitude compared with less severe media, while oil (and properly tuned polymer quenchants) can reduce residual stresses. Use oil when the steel can still fully harden at that cooling rate, and reserve water for steels and geometries that truly require it to meet the critical cooling rate, accepting the higher crack risk and controlling geometry and process more tightly.

References

1. Uddeholm. Heat Treatment of Tool Steel (PDF, 2024). https://www.uddeholm.com/app/uploads/sites/216/2024/05/Tech-Uddeholm-Heat-treatment-EN.pdf
2. Samuel, A., and Prabhu, K. N. Residual Stress and Distortion during Quench Hardening of Steels: A Review. Journal of Materials Engineering and Performance (2022). https://link.springer.com/content/pdf/10.1007/s11665-022-06667-x.pdf
3. Herring, D. What Is Quench Cracking and How Can It Be Prevented? Heat Treat Today (Aug 26, 2025). https://www.heattreattoday.com/what-is-quench-cracking-and-how-can-it-be-prevented/
4. Mudawar, I. (Purdue University). The Boiling Advantage (film boiling and vapor blanket explanation). https://engineering.purdue.edu/mudawar/IECA/boiling-advantage/
5. Quaker Houghton. Heat Treatment Brochure (quench definition and vapor blanket stage control via additives). https://home.quakerhoughton.com/wp-content/uploads/2023/09/bro_heat_treatment_overview_EN_AM.pdf
6. Bodycote. Martempering/Marquenching (process details and benefits). https://www.bodycote.com/what-we-do/precision-heat-treatment/hardening-and-tempering-atmosphere-vacuum/martempering-marquenching/
7. MacKenzie, D. S. Back to basics: Martempering to reduce distortion. Gear Solutions (May 15, 2019). https://gearsolutions.com/departments/hot-seat/back-to-basics-martempering-to-reduce-distortion/
8. Amatanweze, K. T., et al. Residual Stress Distribution, Distortion, and Crack Initiation in Conventional and Intensive Quench Practices. Journal of Materials Engineering and Performance (2023). https://link.springer.com/content/pdf/10.1007/s11665-023-08985-0.pdf