M35 Steel (HSS): Properties and Key Applications

M35 steel, classified as a high-speed steel (HSS), is primarily utilized in the manufacturing of cutting tools and dies. This steel grade is known for its excellent hardness, wear resistance, and ability to retain its hardness at elevated temperatures, making it a preferred choice for high-performance applications. M35 steel is an alloyed tool steel that typically contains significant amounts of tungsten and cobalt, which enhance its properties.

Comprehensive Overview

M35 steel is classified as a high-speed steel, specifically a cobalt high-speed steel, which is designed to withstand high temperatures and maintain hardness during cutting operations. The primary alloying elements in M35 include:

• Tungsten (W): Enhances hardness and wear resistance.
• Cobalt (Co): Improves high-temperature performance and toughness.
• Molybdenum (Mo): Contributes to strength and hardenability.

The unique combination of these alloying elements results in a steel that exhibits exceptional hardness, typically reaching values of 62-65 HRC after heat treatment. M35 steel also demonstrates good toughness, making it less prone to chipping and breaking under stress.

Advantages:
- High Hardness: Retains hardness at elevated temperatures, making it suitable for high-speed cutting applications.
- Wear Resistance: Excellent resistance to wear, extending tool life.
- Versatility: Can be used for various cutting tools, including drills, taps, and milling cutters.

Limitations:
- Cost: Higher alloy content leads to increased material costs compared to standard tool steels.
- Machinability: More challenging to machine than lower alloy steels due to its hardness.
- Brittleness: While tough, it can be brittle if not properly heat-treated.

M35 steel holds a significant position in the market for high-speed steels, often used in industries requiring precision cutting tools. Its historical significance lies in its development as a response to the need for materials that could withstand the rigors of high-speed machining.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	T31535	USA	Closest equivalent to M2 with cobalt addition
AISI/SAE	M35	USA	Commonly used in tool manufacturing
ASTM	A600	USA	Specification for high-speed steels
DIN	1.3243	Germany	Minor compositional differences
JIS	SKH55	Japan	Similar properties, but with different heat treatment recommendations

The differences between M35 and its equivalents, such as M2 or SKH55, often lie in the cobalt content and heat treatment processes, which can significantly affect performance in high-speed applications. For instance, the addition of cobalt in M35 enhances its ability to withstand thermal fatigue, making it preferable for specific high-performance applications.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	0.90 - 1.05
W (Tungsten)	5.50 - 6.75
Mo (Molybdenum)	4.00 - 5.00
Co (Cobalt)	4.00 - 5.00
Cr (Chromium)	3.75 - 4.50
Mn (Manganese)	0.20 - 0.40
Si (Silicon)	0.20 - 0.40

The primary role of the key alloying elements in M35 steel includes:

• Carbon: Provides hardness and strength through the formation of carbides.
• Tungsten: Enhances wear resistance and maintains hardness at high temperatures.
• Cobalt: Improves toughness and thermal stability, allowing for better performance in high-speed applications.

Mechanical Properties

Property	Condition/Temper	Typical Value/Range (Metric)	Typical Value/Range (Imperial)	Reference Standard for Test Method
Tensile Strength	Annealed	850 - 1000 MPa	123 - 145 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	600 - 800 MPa	87 - 116 ksi	ASTM E8
Elongation	Annealed	5 - 10%	5 - 10%	ASTM E8
Hardness (HRC)	Quenched & Tempered	62 - 65 HRC	62 - 65 HRC	ASTM E18
Impact Strength	Room Temperature	20 - 30 J	15 - 22 ft-lbf	ASTM E23

The combination of high tensile and yield strength, along with excellent hardness, makes M35 steel suitable for applications involving significant mechanical loading and structural integrity requirements. Its ability to withstand high temperatures without losing hardness is particularly beneficial in high-speed machining environments.

Physical Properties

Property	Condition/Temperature	Value (Metric)	Value (Imperial)
Density	Room Temperature	8.2 g/cm³	0.297 lb/in³
Melting Point/Range	-	1400 - 1450 °C	2552 - 2642 °F
Thermal Conductivity	Room Temperature	25 W/m·K	14.5 BTU·in/h·ft²·°F
Specific Heat Capacity	Room Temperature	460 J/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	Room Temperature	0.000015 Ω·m	0.000015 Ω·in

Key physical properties such as density and thermal conductivity are crucial for applications where weight and heat dissipation are concerns. The relatively high density of M35 steel contributes to its robustness, while its thermal conductivity allows for effective heat management during cutting operations.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Chlorides	3-5	20-60 °C (68-140 °F)	Fair	Risk of pitting
Acids	10-20	20-40 °C (68-104 °F)	Poor	Susceptible to corrosion
Alkaline Solutions	5-10	20-60 °C (68-140 °F)	Fair	Moderate resistance

M35 steel exhibits moderate resistance to corrosion, particularly in chloride environments, where it may be susceptible to pitting. Compared to other high-speed steels like M2, M35's cobalt content provides slightly better resistance to oxidation at elevated temperatures, but it is still not recommended for applications in highly corrosive environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	600 °C	1112 °F	Retains hardness at high temps
Max Intermittent Service Temp	650 °C	1202 °F	Short-term exposure only
Scaling Temperature	700 °C	1292 °F	Risk of oxidation beyond this temp
Creep Strength considerations	500 °C	932 °F	Begins to lose strength

M35 steel performs well at elevated temperatures, maintaining its hardness and strength. However, prolonged exposure to temperatures above 600 °C can lead to oxidation and scaling, which may compromise its integrity.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
MIG	ER70S-6	Argon + CO2	Preheat recommended
TIG	ER80S-D2	Argon	Requires careful control
Stick	E7018	-	Not recommended for thick sections

M35 steel is generally not recommended for welding due to its high hardness and potential for cracking. Preheating and post-weld heat treatment are essential to minimize these risks. The choice of filler metal is critical to ensure compatibility and maintain mechanical properties.

Machinability

Machining Parameter	M35 Steel	AISI 1212	Notes/Tips
Relative Machinability Index	60	100	More difficult to machine
Typical Cutting Speed (Turning)	30-40 m/min	80-100 m/min	Use carbide tools

M35 steel has a lower machinability index compared to more common steels like AISI 1212, making it more challenging to machine. Optimal cutting speeds and tooling are essential to achieve desired results without excessive wear.

Formability

M35 steel is not particularly suited for forming processes due to its high hardness and brittleness. Cold forming is generally not feasible, while hot forming may be possible with careful temperature control. Work hardening can occur, necessitating consideration of bend radii and forming techniques.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	800 - 850 °C (1472 - 1562 °F)	1 - 2 hours	Air	Reduce hardness, improve machinability
Hardening	1200 - 1250 °C (2192 - 2282 °F)	30 - 60 minutes	Oil	Increase hardness
Tempering	550 - 600 °C (1022 - 1112 °F)	1 hour	Air	Reduce brittleness, increase toughness

The heat treatment of M35 steel involves austenitizing, quenching, and tempering to achieve the desired hardness and toughness. The metallurgical transformations during these processes significantly impact the microstructure, leading to the formation of fine carbides that enhance wear resistance.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Aerospace	Cutting tools for aircraft manufacturing	High hardness, wear resistance	Precision and durability
Automotive	Drill bits and taps	Retains hardness at high temperatures	Efficiency in machining
Metalworking	Milling cutters	Toughness and wear resistance	Long tool life

Other applications include:
- Tooling: M35 is widely used for manufacturing high-performance tooling due to its hardness and wear resistance.
- Machining: Ideal for high-speed machining operations where tool longevity is critical.

M35 steel is chosen for these applications due to its ability to maintain performance under extreme conditions, ensuring precision and efficiency in manufacturing processes.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	M35 Steel	M2 Steel	HSS Grade X	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High hardness	Good toughness	Excellent wear resistance	M35 offers better high-temp performance
Key Corrosion Aspect	Moderate resistance	Fair resistance	Good resistance	M35 is less resistant to acids
Weldability	Poor	Fair	Poor	M35 requires careful welding techniques
Machinability	Low	Moderate	Moderate	M35 is harder to machine
Formability	Poor	Good	Fair	M35 is not suitable for forming
Approx. Relative Cost	High	Moderate	Moderate	M35's alloying elements increase cost
Typical Availability	Moderate	High	High	M35 may be less common than M2

When selecting M35 steel, considerations include its cost-effectiveness, availability, and specific application requirements. While it offers superior performance in high-speed applications, its higher cost and lower machinability may necessitate careful evaluation against alternatives like M2 steel or other high-speed steels.

In conclusion, M35 steel is a high-performance material that excels in demanding applications, particularly in the manufacturing of cutting tools. Its unique properties, while advantageous, also require careful handling and processing to maximize its potential.