AWS規格 D8.9M 4th Edition, 2022: Test Methods for Evaluating the Resistance Spot Welding Behavior of Automotive Sheet Steel Materials

Description

This document contains several standardized test methods designed to evaluate the resistance spot welding behavior of coated and uncoated sheet steels in a laboratory environment. The test methods allow for uniform testing of automotive sheet steels to determine the following:

(1) The effect of the interaction between a sheet steel’s coating (metallic, nonmetallic, or both) and the welding electrodes on electrode deterioration and weld size/quality behavior over an extended number of welds
(2) Current levels and current range
(3) Mechanical properties of welds at different weld sizes and hold times
(4) Metallurgical and hardness properties of welds
(5) The effects of sheet lubricants, conductive paints, or other surface treatment on current break-through
(6) The robustness of a grade of steel to variations in key welding process variables

The test methods are intended for application in a laboratory environment to characterize certain aspects of the welding behavior of sheet steel products under controlled experimental conditions. They are not intended to simulate production welding practices or to predict welding performance of a given grade of steel in production operations. The test methods and parameters are designed to be used for sheet steels ranging in thickness from 0.5 mm to 4.0 mm. The tests may be used for sheet steel materials of all yield strengths typically used in automotive applications. In the endurance test, a minimum of two tests per material is recommended to obtain an accurate assessment. The endurance test was developed for low strength (tensile strength less than or equal to 300 MPa) coated steels of lighter gauges. This test is normally used for coated steels with tensile strength less than or equal to 500 MPa and gauges less than 1.2 mm. The weld property tests are designed primarily for gauges of 1.2 mm and heavier.

The specific type and number of tests required shall be at the discretion of the specifying party. While the test methods were developed to compare the welding behavior of different steel grades, they can be judiciously applied to evaluate other aspects of welding behavior. Not all of the above mentioned tests are required to establish the resistance spot welding behavior of a given grade of steel. The current level/current range testing and weld property testing together generally provide a good indication of the resistance spot welding behavior, whereas the DoE testing is intended to examine the robustness of a given grade of high strength steel to variations in key process variables.

The specific tests required to establish the resistance spot welding behavior of a given grade of steel should be agreed upon by the steel supplier (or test laboratory) and the automotive company (or buyer of the steel).

A commentary is provided in Annex E on alternative test methods and is intended to serve as a guide should the users of this document desire to utilize alternative test methods for the weld characterization of sheet steels. Because the weldability evaluation procedures in alternative methods deviate from the procedure described in this document, the use of the alternative test methods may add to the complexity and cost of the material evaluation. The type of evaluations required, and any deviations required to perform them, should be agreed upon between the party for whom the testing is to be performed and the testing laboratory.

Resistance spot welding behavior is dependent upon a wide variety of interacting material and weld process factors. To obtain repeatable and reproducible performance data, it is imperative that all experimental variables and the way a test is conducted be closely controlled. Therefore, the instructions for the various sampling and testing procedures in this manual are stated in mandatory language and should be followed as closely as possible. Deviations from the prescribed procedures or test methods, permissible with the consent of the party for whom the testing is performed, have to be noted and reported.

For the purpose of determining appropriate welding parameters and tests required, steel grades have been classified into four groups based on their minimum tensile strength. These groups are listed in Table 1. Also provided in Table 1 are examples of typical steel grades that fall into each of these four groups.