Microstructure – A Short Definition:
Steel microstructure refers to the existing described order and arrangement of observable constituents of steel Material at any location throughout its section or thickness.
How to Observe Microstructures:
The most frequent use of the term microstructure refers to the look of polished and etched specimens taken from selected material samples and observed through a microscope. Technicians viewing microstructures may record them by photographic methods for further study, or to use for example.
How Does Microstructure Form?
Steel microstructure results from: melting practice, Chemistry, forming method, gross mass and shape, and the selection and effectiveness of any Heat Treating performed. Typically the microstructure will vary from location to location, throughout any thick steel section, as a result of cooling rate after forming, or as a response to heat treat cooling rate and any tempering if performed.
Microstructure – Relationship to Material Strength and Ductility:
Created by the selection of steel chemistry and all subsequent processing methods used to make a final steel product, the resulting microstructure will determine all aspects of strength and ductility of any object made from any Heat Lot (melted furnace load) of steel.
Microstructure and Material Performance:
Material scientists almost always study microstructure in order to discover the cause of unexpected material failure. It seems that if the microstructure matters so much in the failure analysis of materials, then it matters as much or more for successful material performance.
Why Material Test Results and Actual Part Properties Vary:
Generally, manufacturers of steel products use mechanical testing of a sample piece of steel material (Qualification Test Coupon) to qualify steel parts made from the same Heat Lot and processed in the same manner. Various Industry Standards specify process controls that attempt to make the comparison between the test sample and production parts meaningful. Again, generally speaking, if the specified mechanical testing of the test sample produced the desired results, examination of the test sample microstructure would also reveal a desired microstructure.
Even with specified controls, variation in forming methods, heat treat equipment variation (principally quench media volume and agitation force), and the decision of the manufacturer as to the material mass and shape before heat treating, will create an opportunity for significant differences in microstructure between test samples and production parts. This difference in microstructure, if realized and determined, would indicate the differences in properties between the recorded results of tests performed on test samples, and properties recorded during later examination of samples taken from equipment units that may have failed in service or otherwise become the subject of study.
Inadvertent Altering of Microstructure/Material Properties:
While heat treating provides a good method to produce a desired microstructure, welding may provide an opportunity to alter that microstructure in an undesirable way.
Besides threaded pipe connections and threaded fasteners, welding has become the most common method of joining steel pieces together, and welding has become the preferred method of joining the forged or cast components of pressure containing or pressure controlling equipment. Various Industry Standards specify requirements for welding procedures, each procedure qualified by a test of a representative sample of specified material, welded by methods required in the documented procedure (including post weld heat treating if required in the procedure). The word representative indicates similarity. This qualification process assumes that later pieces of material falling into the same standard material category and size range, using the same qualified welding procedure, will respond to heat inputs in a similar way.
In the real world, standard industrial steels like 4130 can have enough variation in chemistry, as allowed by the tolerance of each chemical element, to fall into informal categories of “lean” or “rich.” Joining “lean” to “rich” material by welding will produce two considerably different Heat Affected Zone microstructures on the two sides of the weld. Perhaps one, or the other, or both of these microstructures will lack any similarity to the qualifying welded sample.
Manufacturers must understand the limitations of reliance on the letter of specifications. Care taken to limit the variation of inputs gives greater uniformity of output.
Care in Manufacturing Assures Reliability of Equipment:
The individual people selecting material and performing work that may affect the properties of that material (read microstructure) must know how to control the variation of inputs in order to control the variation of outputs. Many of the specified steps in welding procedures call for process inputs that inspectors cannot measure or evaluate after the completion of a weld; these inputs include: preheat, maintenance of interpass temperature, voltage and amperage settings, gas (if used), and the specific electrode used.
The manufacture of equipment that will not fail requires personal integrity on the part of everyone involved in work that may affect material properties. These individual people must have a dedication to the study of materials and the processes they use in connection with these materials. These people must realize that unexamined experience teaches very little, so in order to advance in ability and performance; they must question, in detail, every variation observed. Because some variation will always occur, people of integrity will observe and eliminate the unexpected bad, and they will take the appropriate action to minimize its recurrence. They will also take the appropriate action to perform and repeat the desirable good.
You can count on the integrity of the people and the performance of the products of WOODCO USA.