Cryogenic Treatment of Steel: Part One

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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