Units for Material Properties, Treatment and Testing: SI Prefixes

SI Prefixes: The Decimal Multipliers

In engineering practice, the preferred prefixes step by factors of 1,000: kilo (10³), mega (10⁶), giga (10⁹) on the large side; milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹) on the small side. Avoid mixing centi-, deci-, deka-, and hecto- except where convention demands them (centimeter, hectare).

Traditional Metric Measures (Still Found in Legacy Documents)

Length:

• 10 millimeters (mm) = 1 centimeter (cm)
• 10 centimeters = 1 decimeter (dm)
• 10 decimeters = 1 meter (m)
• 1,000 meters = 1 kilometer (km)

Volume:

• 10 milliliters (ml) = 1 centiliter (cl)
• 10 centiliters = 1 deciliter (dl)
• 10 deciliters = 1 liter (l)
• 1 liter = 1 cubic decimeter

Mass:

• 10 milligrams (mg) = 1 centigram (cg)
• 10 centigrams = 1 decigram (dg)
• 10 decigrams = 1 gram (g)
• 1,000 grams = 1 kilogram (kg)
• 1,000 kilograms = 1 metric ton (t)

The Conversion Tables That Save Careers

the practitioner spent his rebuilding year creating conversion reference sheets that he laminated and posted at every inspection station in the plant. Here are the critical ones.

Length Conversions

Worked Example — Converting 0.4476 inches to millimeters:

0.4    inch = 10.16000 mm
0.04   inch =  1.01600 mm
0.007  inch =  0.17780 mm
0.0006 inch =  0.01524 mm
─────────────────────────
0.4476 inch = 11.36904 mm

Worked Example — Converting 84.9 mm to inches:

80.  mm = 3.14961 in.
 4.  mm = 0.15748 in.
 0.9 mm = 0.03543 in.
────────────────────
84.9 mm = 3.34252 in.

Surface Finish: Microinch ↔︎ Micrometer Conversion

Surface roughness is specified in microinches (µin.) in U.S. practice and micrometers (µm) in SI. The conversion factor is:

$$1 \text{ µin.} = 0.0254 \text{ µm} \quad \text{(exact)}$$

$$1 \text{ µm} = 39.37 \text{ µin.}$$

Area Conversions

Volume Conversions

Weight/Mass Conversions

Force Conversions

Pressure and Stress Conversions

the practitioner's critical conversion: The bolt that failed was rated at 80 kgf/mm². To convert to psi: $80 \times 14.22334 \times 100 = 113,787 \text{ psi}$. The required specification was 120,000 psi. A 5.2% shortfall—invisible to someone who doesn't know the conversion.

Energy and Work Conversions

Power Conversions

Power and Heat Equivalents (Quick Reference):

• 1 horsepower = 746 watts = 33,000 ft·lb/min = 550 ft·lb/s = 42.4 Btu/min
• 1 kilowatt = 1,000 watts = 1.34 horsepower = 3,415 Btu/hr
• 1 Btu = 1,052 watt-seconds = 778 foot-pounds
• 1 foot-pound = 1.36 joules

Torque Conversions

Density Conversions

Temperature Conversions

The formulas:

$$°C = \frac{5}{9} \times (°F - 32)$$

$$°F = \frac{9}{5} \times °C + 32$$

$$\text{Kelvin} = °C + 273.16$$

$$\text{Rankine} = °F + 459.7$$

Absolute zero = −273.16°C = −459.7°F

Thermal Energy:

• 1 Btu = the quantity of heat required to raise 1 pound of pure water 1°F
• 1 kilogram calorie = heat to raise 1 kg of water 1°C = 3.968 Btu
• 1 kilojoule = 0.9478 Btu
• Mechanical equivalent of heat (Joule's constant) = 778 foot-pounds per Btu

Material Properties — The Numbers That Govern Design

the practitioner's second lesson was harder than the first. After mastering unit conversions, he realized that the numbers being converted were meaningless unless he understood what they measured. A steel casting with a tensile strength of 120,000 psi is just a number unless you understand what tensile strength is, how it's measured, and what it means for your application.

The Core Mechanical Properties

Every engineering material is characterized by a set of mechanical properties that determine its suitability for a given application. These properties are measured through standardized tests, and the results are expressed in the units we just covered.

Tensile Strength

Definition: The maximum stress a material can withstand while being stretched or pulled before breaking.

Units: psi (lb/in²), ksi (1,000 lb/in²), MPa, or kgf/mm²

How it's measured: A standardized test specimen is pulled in a tensile testing machine until it fractures. The maximum load divided by the original cross-sectional area gives the ultimate tensile strength (UTS).

$$\sigma_{UTS} = \frac{F_{max}}{A_0}$$

Where $F_{max}$ is the maximum force and $A_0$ is the original cross-sectional area.

Yield Strength (Yield Point)

Definition: The stress at which a material begins to deform permanently (plastically). Below this stress, the material returns to its original shape when the load is removed.

Why it matters more than tensile strength in design: Engineers design structures to stay below yield strength—not ultimate strength. A bridge that permanently sags has failed, even if it hasn't snapped.

Elongation

Definition: The percentage increase in length of a test specimen at fracture, measured over a standard gauge length (typically 2 inches).

$$\text{Elongation} = \frac{L_f - L_0}{L_0} \times 100\%$$

What it tells you: Ductility. High elongation means the material can absorb energy and deform before fracturing—critical for impact resistance.

Hardness

Definition: Resistance to surface indentation or penetration.

Hardness is the single most frequently tested mechanical property in manufacturing because it can be measured quickly, non-destructively, and with portable equipment. There are three major scales:

Hardness Testing: Brinell, Rockwell, and Vickers

Brinell Hardness Number (BHN or HB)

Method: A hardened steel or tungsten carbide ball (typically 10 mm diameter) is pressed into the material surface under a known load (typically 3,000 kgf for steel). The diameter of the resulting indentation is measured.

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

Where:

• $P$ = applied load in kgf
• $D$ = diameter of ball in mm
• $d$ = diameter of indentation in mm

Range: Typically 100–750 BHN for steels Best for: Castings, forgings, and raw materials with coarse or uneven grain structures

Rockwell Hardness (HR)

Method: A diamond cone (Brale) or steel ball indenter is pressed into the material under a minor load (10 kgf), then a major load is applied. The depth of penetration under the major load, relative to the minor load position, gives the hardness number.

Key Scales:

Important corrections: When testing spherical surfaces (like steel balls), corrections must be added to the reading. For example, a chrome alloy steel ball reading 60 HRC on a 1/4" diameter ball requires adding +2.9 correction points; on a 1" ball, the correction is only +0.4.

Vickers Hardness (HV or VHN)

Method: A square-base diamond pyramid indenter is pressed into the material under a known load. The diagonals of the resulting square indentation are measured.

Advantage: A single continuous scale from the softest to the hardest materials. Micro-Vickers testing uses very small loads for thin layers and coatings.

Approximate Hardness Conversions

the practitioner's rule of thumb: For carbon and alloy steels in the 120–400 BHN range, the approximate tensile strength in psi equals the BHN multiplied by 500. This is a rough estimate—always verify with actual test data for critical applications.

Material Properties of Carbon and Alloy Steel Castings

the practitioner's next deep dive was into the materials he inspected every day. Steel castings are the backbone of heavy equipment manufacturing, and their properties depend entirely on composition and heat treatment.

Carbon Steel Castings

Carbon steel castings are classified by carbon content:

Low-Carbon (< 0.20% C):

• Carbon: 0.16–0.19%
• Manganese: 0.50–0.85%
• Silicon: 0.25–0.70%
• Tensile strength (annealed): 40,000–70,000 psi

Medium-Carbon (0.20–0.50% C):

• Manganese: 0.50–1.00%
• Silicon: 0.20–0.80%
• Tensile strength: 65,000–105,000 psi (depends on heat treatment)

High-Carbon (> 0.50% C):

• Manganese: 0.50–1.00%
• Silicon: 0.20–0.70%
• Tensile strength (fully annealed): 95,000–125,000 psi

Mechanical Properties of Steel Castings by Grade

Engineering Grades of Low Alloy Steel Castings

Ductile (Nodular) Cast Iron

Ductile cast iron contains graphite in spheroidal form rather than flakes, producing dramatically different properties:

The grade nomenclature is intuitive: Grade 60-40-18 means minimum 60,000 psi tensile, 40,000 psi yield, and 18% elongation in 2 inches.

Improvement method and result

After the bolt failure, the practitioner became fascinated with heat treatment—the process that transforms the same chunk of steel from soft and ductile to hard and wear-resistant. He learned that most material failures trace back to either wrong material selection or wrong heat treatment.

The Four Primary Heat Treatment Processes

. Annealing

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

Process: Heat the steel to above its critical temperature, hold it there to allow the microstructure to fully transform to austenite, then cool it very slowly (typically in the furnace).

Result: Soft, ductile material with low hardness. Typical BHN: 120–160 for carbon steels.

. Normalizing

Purpose: Produce a uniform, fine-grained structure with moderate strength and hardness.

Process: Heat to above the critical temperature (same as annealing), but cool in still air rather than in the furnace.

Result: Slightly harder and stronger than annealed material. Typical BHN: 130–200. Widely used for structural steel castings.

. Quenching and Tempering

Purpose: Achieve high hardness and strength, then adjust toughness by tempering.

Process:

1. Quenching: Heat to above the critical temperature, then cool rapidly by immersion in water, oil, or brine. This produces martensite—extremely hard and brittle.
2. Tempering: Reheat to a lower temperature (300–1200°F depending on the desired balance of hardness and toughness), hold, then cool. This reduces brittleness while retaining most of the hardness.

Result: The most versatile heat treatment. By adjusting tempering temperature, engineers can dial in a precise balance:

. Case Hardening (Carburizing, Nitriding)

Purpose: Create a hard, wear-resistant surface while maintaining a tough, ductile core.

Carburizing: The steel is heated in a carbon-rich environment. Carbon diffuses into the surface, increasing its carbon content. After carburizing, the part is quenched to harden the high-carbon case while the low-carbon core remains relatively soft.

Nitriding: The steel is heated in a nitrogen-rich environment (typically ammonia gas) at lower temperatures (925–1050°F). No quenching is required—the nitride compounds formed at the surface are inherently hard (up to 90–94 HRA for Nitralloy steels).

Case-Hardening vs. Through-Hardening Steels: A Decision Framework

Steels for Gear Applications: A Complete Reference

The gear industry provides one of the best-documented examples of heat treatment selection:

Case-Hardening Gear Steels:

Through-Hardening Gear Steels:

Forged and Rolled Carbon Steels for Gears (Chemical Composition):

The pinion-harder-than-gear principle: Making the pinion harder than the mating gear equalizes wear rates. The pinion, with fewer teeth, does more work per tooth. The harder pinion corrects minor gear tooth errors through initial wear, then burnishes the gear surface. For high-ratio applications without severe shock loads, a casehardened pinion running with an oil-treated gear (machined after treatment) is an excellent combination.

Hard-Facing Materials — When Surfaces Must Survive Extreme Abuse

In his second year of recovery, the practitioner was assigned to a project involving mining equipment—rock crushers, conveyor screws, and valve seats that wore through in weeks. This introduced him to the world of hard-facing alloys: specialized materials welded or sprayed onto working surfaces to dramatically extend service life.

Hard-Facing Material Selection Guide

Austenitic Manganese Steel: The Self-Hardening Wonder

This material deserves special attention because of its extraordinary property: it work-hardens under impact.

• As-deposited hardness: 170–230 BHN
• After cold working: 450–550 BHN surface hardness
• Yield strength in compression: Low initially, but rises rapidly with deformation until plastic flow ceases

This means the harder you hit it, the harder it gets. It's used on rock-crushing equipment, railway frogs and crossings, and any surface that must absorb repeated heavy impact.

The catch: Manganese steel becomes brittle when reheated above 500–600°F. It cannot be machined with ordinary tools—grinding wheels on specially adapted machines must be used.

Cobalt-Base Alloys: The High-Temperature Champions

Cobalt-chromium alloys (Stellite family) retain hardness at elevated temperatures better than any other common hard-facing material:

• At room temperature: 38–58 HRC (depending on grade)
• After heating: Temporary hardness reduction, but returns to approximately original hardness upon cooling
• Above 1,200°F: Their advantages over other hard-facing materials become decisive

These alloys are used on exhaust valve seats in aircraft, truck, and bus engines; valve trim in steam engines; pump shafts subject to corrosion and erosion.

Nondestructive Testing (NDT) — Verifying Without Destroying

The final pillar of the practitioner's education was testing. Every material property number in this guide is meaningless unless you can verify that a finished part actually meets its specification. Destructive testing (cutting tensile test bars, breaking hardness test samples) provides data but destroys the part. Nondestructive testing allows inspection of actual production components.

The Major NDT Methods

NDT Symbol System (Per ANSI/AWS 2.4)

NDT requirements on engineering drawings use a standardized symbol system. The testing symbol consists of:

• Reference line (horizontal line)
• Arrow (connecting to the part being tested)
• Basic testing symbol (abbreviation placed on the reference line)
• Arrow side vs. other side placement (same convention as welding symbols)
• (N) = number of tests
• Length of section to be tested (shown to the right of the symbol)

Tests on the arrow side of the part are indicated by the symbol on the side of the reference line toward the reader. Tests on the other side are indicated on the far side of the reference line.

Ultrasonic Bolt Tension Measurement

A specialized NDT application critical for bolted joint integrity:

A sound pulse is generated at one end of a bolt, travels its length, bounces off the far end, and returns. The transit time depends on bolt length, material, temperature, and stress level. By comparing the loaded transit time to an unstressed baseline, the system can compute stress, load, or elongation.

Requirements:

• Both bolt ends must be finished square to the bolt axis
• Accuracy is comparable to strain gauge methods
• Limited by sonic velocity variations between bolts of the same material

The turn-of-nut method (alternative):

$$\theta = \frac{360 \times F_t \times L_B}{E \times l}$$

Where $\theta$ = turn angle in degrees, $F_t$ = target preload, $L_B$ = effective bolt length, $E$ = modulus of elasticity, and $l$ = thread lead.

Engineering Abbreviations — The Language of Precision

the practitioner discovered that miscommunication often hid not in conversion errors but in abbreviation confusion. Here is the definitive reference:

Standard Engineering Abbreviations

IEC (International Electrotechnical Commission) Symbols

Best Practices for Using Abbreviations

• Abbreviate only with numerals: Write "several inches," not "several in."
• Never begin a sentence with a numeral followed by an abbreviation
• Avoid pound signs for abbreviations: use "lb," not "#"
• Spell it out when in doubt. Abbreviations should be used sparingly and only where their meaning is unmistakable.

Ohm's Law and Electrical Relations — The Circular Reference

For engineers who cross between mechanical and electrical systems, the Ohm's Law circular reference is an indispensable tool:

The Four Variables

The Twelve Formulas

Finding Voltage: $$V = I \times R \quad \text{or} \quad V = \frac{W}{I} \quad \text{or} \quad V = \sqrt{W \times R}$$

Finding Current: $$I = \frac{V}{R} \quad \text{or} \quad I = \frac{W}{V} \quad \text{or} \quad I = \sqrt{\frac{W}{R}}$$

Finding Resistance: $$R = \frac{V}{I} \quad \text{or} \quad R = \frac{V^2}{W} \quad \text{or} \quad R = \frac{W}{I^2}$$

Finding Power: $$W = V \times I \quad \text{or} \quad W = I^2 \times R \quad \text{or} \quad W = \frac{V^2}{R}$$

Fastener Strength Grades — The Final Layer of Material Intelligence

The bolt that started the practitioner's journey was a fastener. So it's fitting that this guide ends with the grading systems that separate a reliable bolt from a catastrophic one.

SAE/ASTM Inch-Series Bolt Grades

ISO Metric Bolt Strength Grades

The ISO system uses a two-digit designation that encodes both tensile strength and yield ratio:

• First digit × 100 = minimum tensile strength in MPa (or × 10 = tensile in kgf/mm²)
• Second digit × 10 = ratio of yield stress to tensile strength (%)

Example: A Grade 8.8 bolt has a minimum tensile strength of 80 kgf/mm² and a permanent set limit (comparable to yield) of 64 kgf/mm². The "8" means 1/10 × 80, and the ".8" means the yield-to-tensile ratio is 80%.

ISO Metric Nut Strength Grades

The nut strength grade is a single number equal to one-tenth of the proof load stress in kgf/mm². The proof load stress corresponds to the minimum tensile strength of the highest bolt grade the nut can safely mate with.

Recommended Bolt and Nut Combinations:

Counterfeit Fastener Detection

Counterfeit fasteners are marked as belonging to a specific grade but fail to meet the standard. They may:

• Be made from the wrong material
• Lack proper heat treatment
• Break unexpectedly at loads below specification

Detection methods include:

• Hardness testing (Rockwell or Brinell)
• Elongation testing
• Ultimate load testing
• Chemical analysis

The only sure way to verify a fastener meets its specification is to test it. Reputable distributors will assist in verification.

Your Action Steps

If you're a machinist or inspector:

1. Print the conversion tables in this guide and post them at your workstation.
2. Never assume metric and inch specifications are "close enough." Calculate the exact conversion.
3. Verify hardness readings with the appropriate scale corrections, especially on curved surfaces.

If you're an engineer or designer:

1. Specify materials with both metric and customary equivalents on critical drawings.
2. Include the heat treatment condition alongside every hardness specification.
3. When specifying fasteners, always reference the grade marking and the standard (SAE, ASTM, or ISO).

If you're a project manager or buyer:

1. Require material test reports (MTRs) for all structural components.
2. Insist on NDT verification for critical joints and welds.
3. Train your team to recognize the difference between grades, and fund testing for any fastener or material whose provenance is uncertain.

What's the most expensive unit conversion error you've ever encountered—or narrowly avoided? The answer might save someone else from repeating it.