What does the grade of a metric bolt mean and how do they compare to SAE grades?

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A. Metric bolts use the specification ISO 898 to specify the required mechanical properties for all the different classes of materials used to make threaded fasteners. In general there are 4 commonly used classes for externally threaded fasteners, 4.8, 8.8, 10.9 & 12.9 The larger the number, the harder the steel Class 4.8 is mild steel, used for machine screws, machine bolts, etc. Class 8.8 is a medium hard steel used for hex cap screws and hex socket screws. Class 10.9 is usually an alloy steel used for hex cap screws. Class 12.9 is used to make hex socket cap screws, set screws, etc. The following table shows the comparison between metric classes and SAE grades.
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