Skip to content 
A property class on a fastener drawing is a promise about behavior. When an engineer specifies grade 10.9, the joint is being designed around a known tensile and yield strength, and the part that arrives has to deliver that number part after part. Reaching a class such as 8.8, 10.9, or 12.9 is therefore less about a single heat and more about a disciplined chain of steps that begins with the wire and ends with verified microstructure.
Heat treatment sits at the center of that chain. It is the stage that turns a properly formed blank into a fastener with the mechanical properties the class demands.
The ISO 898 designation is a compact code for mechanical performance. In a class such as 8.8, the first digit multiplied by one hundred gives the approximate minimum tensile strength in mega pascals, so 8.8 points to roughly 800 MPa. The second digit, read as a fraction of ten, gives the ratio of yield to tensile strength, so the 0.8 means the yield is about eighty percent of the tensile value.
Applied across the common grades, 8.8 reaches about 800 MPa tensile with a 640 MPa yield, 10.9 reaches about 1000 MPa tensile with a 900 MPa yield, and 12.9 reaches about 1200 MPa tensile with a 1080 MPa yield. These are not arbitrary labels. They are the targets that the material chemistry and the heat treatment must hit together, because no amount of furnace work will compensate for the wrong starting steel.
The workhorse route to these strengths is quench and temper. The fastener is heated into the austenite range, typically around 840 to 880 degrees Celsius depending on the steel, and then cooled rapidly in oil, water, or polymer. That sudden quench transforms the structure to martensite, a very hard but brittle phase.
Martensite on its own would crack under load, so the part is tempered at a lower temperature to let some of that hardness trade for toughness. The tempering temperature is the control knob. A higher temper gives more ductility and less strength, while a lower temper holds strength at the cost of toughness. Hitting a class consistently means holding that temper window tight, because a few degrees of drift moves the final properties.
The grades separate mainly by steel chemistry and by how aggressively the temper is set. Grade 8.8 is usually made from a medium carbon steel in the 0.35 to 0.45 percent carbon range and brought to property through quench and temper. Grade 10.9 steps up to a medium carbon alloy steel with elements such as chromium or molybdenum that improve hardenability, and the temper is controlled to land the higher strength without losing toughness.
Grade 12.9 demands the most care. It uses alloy steel with strong hardenability and a narrow temper window, because the line between full strength and unacceptable brittleness is thin at this level. Achieving bolt strength class at 12.9 is therefore as much a quality-system task as a metallurgical one, since the margin for process variation is small.
Heat treatment shapes properties, not geometry. The part that enters the furnace must already be the right shape, which is why cold heading precedes it. The former upsets and extrudes the wire into a net-shape blank at several hundred parts per minute, and that blank then carries the correct head and shank into the quench.
Cold heading also helps the final result. Because the process preserves grain flow through the head and underhead fillet, the formed blank starts from a stronger base than a machined one would. Heat treatment cold formed parts therefore benefits from both the grain structure left by forming and the martensite set by the furnace. One caution follows from this order: the heating can decarburize the surface, so atmosphere control during heat treatment matters for the parts that cold heading prepared.
Two failures dominate when the process drifts. Quench cracking appears when the cooling is too aggressive for the section, when inclusions or prior cold work concentrate stress, or when the part geometry varies in thickness. Hydrogen embrittlement is the other, and it is especially dangerous for high-strength grades. It enters during acid pickling or electroplating after heat treatment, then settles in the steel and can cause delayed fracture, which is why plated 10.9 and 12.9 fasteners are baked to drive the hydrogen out.
Decarburization at the surface and distortion from uneven cooling round out the usual suspects. Each one is manageable, yet each one erodes the consistency that a property class requires, so the furnace and the plating line deserve the same attention as the forming cell.
Verification rests on a few measurements that together confirm the part meets its class. Hardness testing with Rockwell gives a fast read on the temper, while proof load testing checks that the fastener holds a defined load without permanent set. Microstructure examination confirms a tempered martensite structure rather than unwanted ferrite or pearlite, and tensile testing on samples anchors the tensile and yield values the class claims.
Consistency comes from running these checks as statistical process control rather than as a one-time gate. When hardness, proof load, and microstructure are tracked across batches, a drift in the furnace shows up before it reaches the customer, which is the only way to claim a grade reliably.
The furnace choice follows the volume. A continuous mesh-belt furnace carries parts through a controlled atmosphere in a steady thermal cycle, which suits the millions of fasteners that a high-rate line produces and gives the repeatability that high grades need. A batch sealed-quench furnace handles mixed sizes and lower volumes where a continuous line would be wasteful.
For heat treatment cold formed parts at scale, the continuous route is usually the better fit because the atmosphere can be held to prevent decarburization and oxidation, and the cycle stays uniform from the first part to the last. Batch processing remains valuable for prototypes, alloys, and small runs where flexibility matters more than throughput.
Hitting grade 8.8, 10.9, or 12.9 consistently is a chain discipline rather than a single operation. The class is set by steel chemistry and locked in by a tightly controlled quench and temper, then proven by hardness, proof load, and microstructure checks. Cold heading earns its place at the front of that chain by delivering the net-shape, grain-preserving blank that the furnace works on.
No. The two address different things. Cold heading shapes the blank and preserves grain flow, while heat treatment sets the mechanical properties. A high grade needs both steps in sequence.
Its very high strength gives hydrogen less energy to initiate a crack, so plated 12.9 parts in particular must be baked soon after plating to remove absorbed hydrogen.
It is the main control, yet it works together with the steel chemistry. The right alloy gives the hardenability, and the temper sets where in the strength range the part lands.
Hardness and proof load tests sample the batch, and microstructure checks confirm the structure. Statistical control across batches catches drift before it affects shipments.
Yes, because the heating in heat treatment can thin the surface carbon layer and lower hardness there. A controlled atmosphere in the furnace is what prevents it.
Zhejiang DongRui supplies high-speed cold formers suited to the work.Explore the full product range and request a quote to see how a high-rate cold former would feed your heat treatment line.
1. Introduction When you manage a high-volume fastener plant or
1. Introduction When a cold heading machine runs at 350–450
Reading time: ~9 min | Categories: Cold Forging Technology, Industrial
Table of Contents Introduction The Fundamental Difference in Material Usage
Optimized by Seraphinite Accelerator
