Cryogenic Treatment of Steel: Part One

Abstract

Cryogenic treatment significantly enhances the service life of steel components through controlled exposure to extremely low temperatures. This methodology, established in aerospace applications over 30 years ago, produces remarkable improvements particularly for wear-dependent materials like tool steels. The success of cryogenic processing depends on precise control of critical parameters including temperature profile, treatment duration, and subsequent tempering practices. This article examines the fundamental principles of cryogenic treatment, compares cold treatment versus deep cryogenic processing, and provides evidence of substantial performance improvements in treated materials as demonstrated by increased wear resistance and extended service life.


Introduction to Cryogenics

The word "cryogenics" derives from the Greek words "Kryos" (meaning cold) and "Genes" (meaning born). According to the Cryogenics Society of America, cryogenic temperatures are defined as those below 120K (-244°F, -153°C). Cryogenic treatment refers to the process of subjecting materials to subzero temperatures (below 0°C) to enhance service life through beneficial morphological changes that occur during treatment.

Originally developed for aerospace applications, cryogenic processing has been utilized for over 30 years to improve the properties of metals. This treatment has proven particularly valuable for extending tool life and enhancing wear resistance in various steel components.

Fundamental Principles of Cryogenic Science

According to thermodynamic laws, absolute zero represents the lowest theoretically achievable temperature, where molecules reach their minimum energy state. Absolute zero equals -273.15°C or -459.67°F, which forms the zero point of the Kelvin thermodynamic temperature scale. In practical terms, the cryogenic region is typically considered to be below approximately 120K (-153°C).

Common gases transform from gaseous to liquid state at atmospheric pressure at specific temperatures known as normal boiling points (NBP), as shown in Table 1. The resulting liquids are called cryogenic liquids or cryogens.

Table 1: Normal boiling points of common cryogenic fluids

Cryogen (K) (°C) (°R) (°F)
Methane 111.7 -161.5 201.1 -258.6
Oxygen 90.2 -183.0 162.4 -297.3
Nitrogen 77.4 -195.8 139.3 -320.4
Hydrogen 20.3 -252.9 36.5 -423.2
Helium 4.2 -269.0 7.6 -452.1
Absolute zero 0 -273.15 0 -459.67

Cold Treatment vs. Cryogenic Treatment

Metal refrigeration treatments fall into two distinct categories: cold treatment and cryogenic treatment. These processes differ significantly in their application parameters and results:

  • Cold Treatment: Typically performed at approximately -120°F (-84°C) where parts are held (soaked) for 1 hour per inch of thickness, then allowed to warm in ambient air.
  • Cryogenic Treatment: Involves a slow cool-down rate of -5°F per minute (-3°C per minute) from ambient to -320°F (-196°C), followed by a soak period of 24 to 72 hours, and gradual warming to ambient temperature. The cryogenically treated parts then undergo a tempering treatment (300°F to 1000°F or 149°C to 538°C) for a minimum of one hour.

Multiple factors impact how sub-zero treatments affect an alloy. Processing parameters such as time, temperature profile, number of repetitions, and tempering practice interact with material factors including prior heat treatment and alloy composition to determine final results. Table 2 outlines three different sub-zero treatment applications.

Table 2: An overview of sub-zero treatment processes for metals

Process Description Parameters Objective
Shrink fitting Overall contraction of metals when cooled allows tight assembly of parts -70 to -120 °C (-90 to -190 °F) until metal is cold throughout Temporary change in size
Cold treatment of steels Complete martensitic phase transformation -70 to -120 °C (-90 to -190 °F) for 1 hour per 3 cm of cross-section - Transformation of retained austenite to martensite - Increase hardness - Dimensional stability
Cryotreatment of steels Cryotreatment temperatures can create sites to nucleate fine carbides that improve wear resistance in tool steels -135 °C (-210 °F) and below for 34 hours or longer Improved wear resistance through carbide precipitation

Figure 1 below illustrates the significant differences in time-temperature cycles among these processes.

Figure 1: Sub-zero process cycle profiles

Performance Improvements Through Cryogenic Treatment

Table 3 demonstrates the average useful life of various tooling components with and without sub-zero treatment. The "Wear Ratio" parameter—defined as the ratio of life after sub-zero treatment divided by average tool life without treatment—quantifies the improvement this process delivers when correctly applied.

Table 3: Examples of tool life improvements using cryotreatment

Tooling Average life before treatment Average life after treatment Wear ratio
5-cm end mills used to cut C1065 steel 64 parts 200 parts 3.07
Hacksaw blades used to cut bosses on M107 shells 4 h 6 h 1.5
Zone punches used on shell casings 64 shells 5,820 shells 82.5
Nosing thread dies used in metal working 225 shells 487 shells 2.1
Copper resistance welding tips 2 weeks 6 weeks 3.0
Progressive dies used in metal working 40,000 hits 250,000 hits 6.25
Blanking of heat treated 4140 and 1095 steel 1,000 pieces 2,000 pieces 2.0
Broach used on a C1020 steel torque tube 1,810 parts 8,602 parts 4.75
Broaching operation on forged connecting rods 1,500 parts 8,000 parts 5.33
Gang milling T-nuts from C1018 steel with M2 cutters 3 bars 14 bars 4.67
AMT-38 cut-off blades 60 h 928 h 15.4

The substantial differences in wear life between parts cold treated at approximately -80°C (-110°F) versus parts cryogenically treated at -190°C (-310°F) using liquid nitrogen have prompted further research into understanding the underlying mechanisms of improved wear resistance.

Table 4 below provides a comparison of wear resistance improvements in different materials after cold treatment versus cryogenic treatment.

Table 4: Percentage increase in wear resistance after cold treatment and cryotreatment

AISI DIN Material Descriptions Wear Resistance at -79°C [%] Wear Resistance at -190°C [%]
Materials that showed improvement
D2 High carbon/chromium steel 316 817
S7 Silicon tool steel 241 503
52100 Bearing steel 195 420
O1 Oil hardening cold work die steel 221 418
A10 Graphite tool steel 230 264
M1 Molybdenum high speed steel 145 225
H13 Hot work tool steel 164 209
M2 Tungsten/molybdenum high speed steel 117 203
T1 Tungsten high speed steel 141 176
CPM 10V Alloy steel 94 131
P20 Mold steel 123 130
440 Martensitic stainless steel 128 121
Materials without improvement
430 Ferritic stainless steel 116 119
303 1.4305 Austenitic stainless steel 105 110
8620 1.6523 Case hardening steel 112 104
C1020 1.0402 0.20% carbon steel 97 98
AQS Gray cast iron 96 97
T2 Tungsten high speed steel 72 92

December, 2014

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