Units for Material Properties, Treatment and Testing: The Two Worlds of Measurement

The Two Worlds of Measurement: Why Systems Exist and Why They Clash

Before the practitioner could help his inspector, he needed the man to understand something fundamental: every number on an engineering drawing is meaningless without its unit. And the unit you choose determines whether your work crosses borders, passes inspection, and keeps people alive.

The English (Imperial/US Customary) System

The system that built the Industrial Revolution. Rooted in human-scale references — the foot, the inch, the pound — it remains deeply embedded in American manufacturing, oil and gas, construction, and aerospace.

Linear Measures:

Weight (Avoirdupois):

Liquid Measure:

Square Measure:

Cubic Measure:

The English system works. But it creates problems the moment you need to scale, subdivide, or convert between dimensions. There is no elegant factor of 10 linking feet to miles or ounces to tons.

That problem is precisely what the metric world set out to solve.

The Metric System and Its Evolution to SI

The French Revolution gave the world more than political upheaval. In the late 18th century, France established the first metric system — a measurement framework built entirely on the decimal principle. Every step up or down is a clean power of 10.

Traditional Metric Measures of Length:

Measures of Weight:

Dry and Liquid Measure:

Key fact: 1 liter = 1 cubic decimeter = the volume occupied by 1 kilogram of pure water at its maximum density temperature.

The beauty of this system is unity. Length, volume, and mass are all linked through water. That single connection eliminated thousands of arbitrary conversion factors.

But the traditional metric system had its own growing pains. Several competing sub-systems emerged: the CGS (centimeter-gram-second), the MKS (meter-kilogram-second, proposed by the practitioner in 1902), and the technical system (meter-kilogram-force-second).

The confusion was real. And it was solved — decisively — in 1960.

The International System of Units (SI): The Universal Language of Engineering

The Conférence Générale des Poids et Mesures (CGPM) — the body responsible for all international matters relating to the metric system — adopted a rationalized, coherent system in 1954 based on the four MKSA units, plus the kelvin and candela. In 1960, this system was formally named the Système International d'Unités, abbreviated SI. In 1971, the 14th CGPM added a seventh base unit: the mole.

This is the system the engineering world speaks today.

The Seven SI Base Units

SI Derived Units You Must Know

The genius of SI is that every other unit is built from these seven. No arbitrary constants. No disconnected scales.

Additional derived quantities used daily in engineering:

The Newton: Why It Changed Everything

Here is where the practitioner's story intersects with history. In the English and traditional metric systems, it was common practice to apply weight units as force units. The kilogram-force and pound-force were introduced as the forces produced by "standard gravity" acting on one kilogram or one pound mass respectively, with gravitational acceleration taken as 9.80665 m/s² (or 32.174 ft/s²).

The problem? Gravitational acceleration varies around the earth. The weight of a given mass varies with location.

The newton eliminates this problem entirely. It is defined as the force that, when applied to a mass of one kilogram, gives it an acceleration of one meter per second squared — completely independent of g.

$$F = m \times a$$

$$1 \text{ N} = 1 \text{ kg} \times 1 \text{ m/s}^2$$

As a result, the factor g disappears from most dynamics formulas. However, in statics — where weight of a body rather than its mass appears — g does appear where it was formerly absent. A mass of W kilograms is equal to a force of W × g newtons, where g ≈ 9.81 m/s².

Horsepower is not used in SI. It is replaced by the watt. The watt — with its multiples (kilowatt) and submultiples (milliwatt) — is the unit for both mechanical and electrical power.

SI Prefixes: The Decimal Multiplier System

SI units form different sizes by multiplying or dividing the base value by powers of 10. Changes are made simply by shifting the decimal point.

Practical examples in manufacturing:

• Surface roughness: measured in micrometers (µm) or microinches (µin.)
• Wire diameter: measured in millimeters (mm) or thousandths of an inch (mils)
• Cutting force: measured in kilonewtons (kN)
• Power output: measured in kilowatts (kW) or megawatts (MW)
• Electrical resistance of plating: measured in milliohms (mΩ)

The Conversion Battlefield: Where Careers Are Won or Lost

Now back to the practitioner. His inspector was staring at two numbers — 43 Rockwell C and 220 Brinell — with no way to bridge them. This is the conversion battlefield, and it is littered with the wreckage of wrong assumptions.

Essential Length Conversions

How to use conversion tables for compound values:

Example 1: Find the millimeter equivalent of 0.4476 inch.

Example 2: Find the inch equivalent of 84.9 mm.

Surface Finish Conversions: Microinches to Micrometers

Surface finish is one of the most critical measurements in precision manufacturing. The conversion factor is:

$$1 \text{ µin.} = 0.0254 \text{ µm}$$

$$1 \text{ µm} = 39.37 \text{ µin.}$$

Example: Convert 23.55 µm to µin.:

Example: Convert 1,375 µin. to µm:

Area Conversions

Volume Conversions

Critical note: The US gallon and UK (Imperial) gallon are not the same. 1 US gallon ≈ 3.785 liters while 1 UK gallon ≈ 4.546 liters. Confusing them on fluid system specifications can cause overflows, underfills, and catastrophic system failures.

Weight/Mass Conversions

Density Conversions

Force Conversions

Pressure and Stress Conversions

Everyday workshop reference: 1 MPa ≈ 145 psi. Commit this to memory and you can estimate any pressure or stress conversion on the fly.

Energy, Work, and Power Conversions

The mechanical equivalent of heat (Joule's constant): 1 Btu = 778.26 foot-pounds. This value connects thermal energy directly to mechanical work and underpins all thermodynamic calculations in engineering.

Temperature Conversions

Temperature conversions are among the most frequent — and most error-prone — calculations in engineering.

$$°C = \frac{5}{9} \times (°F - 32)$$

$$°F = \frac{9}{5} \times °C + 32$$

$$K = °C + 273.16$$

$$°R = °F + 459.7$$

Absolute zero — the lowest theoretically possible temperature — is −273.16°C or −459.7°F. Temperature reckoned from absolute zero is called absolute temperature. In Celsius, this is expressed as degrees Kelvin (K). In Fahrenheit, as degrees Rankine (°R).

Quick Reference Points:

Torque and Moment Conversions

Moment of Inertia and Section Modulus

Engineering Abbreviations: The Shorthand That Keeps Drawings Clear

Every engineering drawing, specification, and report relies on standardized abbreviations. Misreading one can be as costly as a wrong conversion.

Best practices for using abbreviations on engineering documents:

• Abbreviate units only when paired with numerals. Write "several inches," but "6 in."
• Never begin a sentence with a numeral followed by an abbreviation.
• Avoid period signs for abbreviations in text — use "lb," not "lb."
• When in doubt, spell it out. Clarity always trumps brevity.

Material Properties: The Numbers That Define What Metal Can Do

the practitioner's bridge story continues. When his inspector asked about 43 Rockwell C versus 220 Brinell, the real question was not about scales. It was about what the metal could actually withstand. Material properties are the engineering facts that separate a part that performs from a part that fails.

The Core Mechanical Properties

Tensile Strength — The maximum stress a material can withstand while being stretched or pulled before breaking. Expressed in psi (pounds per square inch), ksi (thousands of psi), MPa (megapascals), or kgf/mm².

Yield Strength (Yield Point) — The stress at which a material begins to deform permanently. Below the yield point, the material will return to its original shape. Above it, permanent deformation occurs.

Elongation — The percentage increase in length of a test specimen at fracture. A measure of ductility. Higher elongation = greater ability to deform without breaking.

Reduction of Area — The percentage decrease in cross-sectional area of a tensile test specimen at the point of fracture. Another measure of ductility.

Hardness — Resistance to indentation or surface deformation. Measured by Brinell, Rockwell, Vickers, or Knoop methods.

Impact Resistance (Toughness) — The ability to absorb energy and deform plastically before fracturing. Measured by Charpy or Izod impact tests.

Fatigue Strength (Endurance Limit) — The maximum stress a material can endure for an infinite number of load cycles without failure.

Shear Strength — The stress required to produce fracture in the plane of the cross-section when the forces act parallel to that plane.

Mechanical Properties of Carbon Steel Castings

This table — compiled by the Steel Founders' Society of America — represents the properties normally expected in production steel castings, classified by tensile strength.

Structural Grades of Carbon Steel Castings:

Engineering Grades of Low-Alloy Steel Castings:

The pattern is clear: As tensile strength and hardness increase, elongation (ductility) decreases. This is the fundamental trade-off in metallurgy. You cannot have maximum hardness AND maximum flexibility in the same material without advanced treatment techniques.

Carbon Steel Castings in the supplied reference

Low-Carbon Steel Castings (< 0.20% C):

• Carbon: 0.16 to 0.19% (typical production range)
• Manganese: 0.50 to 0.85%
• Silicon: 0.25 to 0.70%
• Phosphorus: 0.05% max
• Sulfur: 0.06% max
• Tensile strength (annealed): 40,000 to 70,000 psi

Medium-Carbon Steel Castings (0.20–0.50% C):

• Manganese: 0.50 to 1.00%
• Silicon: 0.20 to 0.80%
• Phosphorus: 0.05% max
• Sulfur: 0.06% max
• Tensile strength: 65,000 to 105,000 psi (depending on heat treatment)

High-Carbon Steel Castings (> 0.50% C):

• Manganese: 0.50 to 1.00%
• Silicon: 0.20 to 0.70%
• Phosphorus & Sulfur: 0.05% max each
• Tensile strength (annealed): 95,000 to 125,000 psi

Ductile (Nodular) Cast Iron Grades — ASTM A 536-80

Ductile cast iron features graphite in spheroidal (ball-like) form rather than flakes. This gives toughness intermediate between cast iron and steel, with shock resistance comparable to mild carbon steel.

Grade nomenclature decoded: The three numbers represent minimum tensile strength, minimum yield strength, and minimum elongation. Grade 60-40-18 = 60,000 psi tensile / 40,000 psi yield / 18% elongation.

Malleable Iron Properties

Malleable iron is produced by annealing (graphitizing) white iron castings. The resulting temper carbon exists as compact, rounded aggregates rather than flakes.

ANSI/ASTM A47-77 Standard Grades:

Pearlitic Malleable Iron (ASTM A 220-79):

Austenitic Manganese Cast Steel

One of the most remarkable engineering alloys. Composition: carbon 1.00–1.40%, manganese 10.00–14.00%, silicon 0.30–1.00%.

In the as-cast condition: quite brittle. But after heating to 1,830–1,940°F and quenching in cold water:

The "self-hardening" phenomenon: When cold worked, austenitic manganese steel surface hardness increases from 180 BHN to as high as 550 BHN. This is why it is used in rock crushers, railway crossings, and mining equipment — the harder you hit it, the harder it gets.

Hardness Testing: The Inspector's Most Powerful Tool

Back to the practitioner's inspector. Understanding hardness scales and their relationships is not optional — it is survival-level knowledge for anyone who approves, rejects, or specifies metal.

Brinell Hardness (BHN)

A hardened steel or tungsten carbide ball (typically 10 mm diameter) is pressed into the material surface under a specified load (typically 3,000 kg for steels, 500 kg for softer metals). The diameter of the resulting indentation is measured, and hardness is calculated as:

$$BHN = \frac{2P}{\pi D \left( D - \sqrt{D^2 - d^2} \right)}$$

Where:

• P = applied load (kgf)
• D = diameter of ball (mm)
• d = diameter of indentation (mm)

Advantages: Good for testing castings, forgings, and rough surfaces. The relatively large indentation averages out local variations.

Limitations: Not suitable for thin materials or case-hardened surfaces (the ball may penetrate through the hard case into the softer core).

Rockwell Hardness (HR)

The most widely used hardness test in manufacturing. It measures the depth of penetration under a specific load, giving a direct reading on a dial or digital display.

Rockwell Scales Most Used in Engineering:

Advantages: Fast, direct reading, minimal surface preparation needed. Well-suited for production environments.

Limitations: Surface must be flat and perpendicular to the indenter. Curved surfaces (like balls) require correction factors.

Vickers Hardness (HV or VHN/DPH)

A diamond pyramid indenter with a 136° angle between opposite faces is pressed into the material. Both diagonals of the square indentation are measured and averaged.

$$HV = \frac{1.854 \times P}{d^2}$$

Where P is the load in kgf and d is the mean diagonal in mm.

Advantages: A single continuous scale covers all materials — from soft lead to hardened steel. Very precise for thin materials and coatings.

Knoop Hardness (HK)

Similar to Vickers but uses an elongated diamond indenter, producing a shallow, elongated indentation. Ideal for brittle materials, thin coatings, and individual microstructural constituents.

Approximate Hardness Conversion Reference

This is the table the practitioner's inspector needed:

the practitioner's answer to his inspector: 220 Brinell ≈ 18 Rockwell C. The reading of 43 Rockwell C corresponds to approximately 400 Brinell — far exceeding the 220 BHN specification. The bars were actually over-hardened, which could mean reduced ductility and potential for brittle fracture under dynamic loading. The inspector needed to reject those bars, not pass them.

Hardness Corrections for Curved Surfaces

When testing spherical surfaces (such as standard metal balls), Rockwell readings must be corrected. Corrections are added to the observed reading.

Heat Treatment of Materials: Controlling the Internal Structure

If measuring units tell you what a material is, heat treatment tells you what it can become. Every mechanical property listed above — tensile strength, yield, hardness, ductility — can be dramatically altered by the controlled application of heat and cooling.

Annealing

Purpose: Soften the material, relieve internal stresses, improve machinability, and refine grain structure.

Process: Heat the steel to a temperature above the upper critical point (typically 1,400–1,650°F / 760–900°C depending on carbon content), hold at temperature to allow complete transformation, then cool very slowly — usually in the furnace.

Result: Maximum softness, maximum ductility, minimum hardness. The microstructure consists of ferrite and pearlite (in carbon steels) in their equilibrium state.

Application: Malleable iron castings are produced by annealing white iron castings. Temperature is slowly increased (over 50+ hours) to 1,650–1,700°F, then slowly cooled (over 60+ hours).

Normalizing

Purpose: Refine grain structure, improve uniformity, moderate strength increase over the annealed condition.

Process: Heat to 100–150°F above the upper critical temperature, then cool in still air (not in the furnace).

Result: Finer, more uniform grain structure than annealing. Slightly higher strength and hardness than the annealed condition.

Application: Standard preliminary treatment for carbon steel castings before further heat treatment. All steel castings for gears must be thoroughly normalized or annealed to eliminate the characteristic structure of unannealed castings.

Quenching (Hardening)

Purpose: Achieve maximum hardness by trapping carbon in a supersaturated solution within the iron crystal lattice (martensite).

Process: Heat to the austenitizing temperature, then cool rapidly by immersion in a quenching medium — water, oil, brine, or polymer solution.

Result: Maximum hardness, but also maximum internal stress, brittleness, and distortion.

Quenching media and their effects:

Critical insight from the practitioner's experience: Thru-hardening steels become distorted when hardened, the amount depending on the steel and quenching medium. For this reason, thru-hardening steels are not suitable for high-speed gearing where noise is a factor, or for gearing where accuracy is paramount — unless grinding of the teeth is practicable.

Tempering (Drawing)

Purpose: Reduce the brittleness of quenched steel while retaining a useful level of hardness.

Process: Reheat the quenched steel to a temperature below the lower critical point (typically 300–1,200°F / 150–650°C), hold at temperature, then cool.

Result: Controlled reduction in hardness with corresponding increase in toughness and ductility. The higher the tempering temperature, the softer and more ductile the steel becomes.