Patent Application
20100037994
This disclosure relates to maraging steel and, more particularly, to an economic method of processing maraging steel to achieve improved mechanical properties.
Maraging steel grades are generally regarded as high strength steels that do not include carbon. The high strength is obtained through thermal processing to form a predominantly martensitic microstructure. For instance, maraging steels are typically solution annealed at a temperature sufficient to form an austenite phase and then cooled to room temperature to transform the austenite phase into the martensitic phase. Several solution annealing steps are typically used to provide a homogenous microstructure. The maraging steel may subsequently be aged at a lower temperature to precipitation harden the maraging steel.
Although the above thermal processing is effective for obtaining the martensitic microstructure, a particular end use of the maraging steel may require other processing steps which alter the final microstructure and mechanical properties of the maraging steel. Thus, between the thermal processing and other processing steps, there may be countless combinations and variables that influence the microstructure and physical properties.
The disclosed methods of processing a workpiece of maraging steel are intended to provide economic processing and improved mechanical properties.
In one example, a method of processing includes receiving a workpiece of maraging steel that has been subjected to thermomechanical processing at an austenite solutionizing temperature and directly aging the workpiece of maraging steel at an aging temperature to form precipitates within a microstructure of the workpiece of maraging steel, without any intervening heat treatments between a thermomechanical processing and the direct aging.
In another aspect, the method may include thermomechanically processing the workpiece of maraging steel at the austenite solutionizing temperature prior to directly aging the workpiece of maraging steel.
In another aspect, a method includes processing a workpiece of maraging steel that has been subjected to thermomechanical processing at an austenite solutionizing temperature to thereby produce a component of maraging steel. The workpiece of maraging steel is directly aged at an aging temperature to form precipitates within the microstructure of the workpiece of maraging steel and establish an ultimate tensile strength of the component of maraging steel that is greater than 265 ksi, without any intervening heat treatments between the thermomechanical processing and the direct aging.
The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.
The workpiece of maraging steel 22 is formed of a maraging steel alloy composition. For example, the maraging steel alloy composition includes 17 wt %-19 wt % of nickel, 8 wt %-12 wt % of cobalt, 3 wt %-5 wt % of molybdenum, 0.2 wt %-1.7 wt % of titanium, 0.05 wt %-0.15 wt % of aluminum, and a balance of iron. As can be appreciated, the composition may vary from the given example composition. In some examples, the given composition consists essentially of the given elements and impurities that do not affect the properties of the alloy or elements that are unmeasured or undetectable in the alloy. For example, the alloy generally does not include carbon, but may include up to about 0.03 wt % of carbon as an impurity. The maraging steel alloy may also be classified by composition, such as by commonly used designations M200, M250, M300, or M350.
The example method 20 includes a thermomechanical processing step 24 that includes working (e.g., plastically deforming) the workpiece of maraging steel 22 at an austenite solutionizing temperature. The process of working may include any suitable type of process, such as rolling, forging, or any forming process. The austenite solutionizing temperature may depend somewhat on the selected composition of the maraging steel. However, in most instances, the austenite solutionizing temperature will be 1500° F.-1750° F. (816° C.-954° C.) to solutionize the compositional components of the maraging steel into a gamma austenite phase microstructrue. The workpiece of maraging steel 22 may be subjected to the austenite solutionizing temperature for the duration of the working process. For instance,
The thermomechanical processing step 24 also includes cooling the workpiece of maraging steel 22 from the austenite solutionizing temperature to transform the gamma austenite phase to an alpha martensite phase. For example, the workpiece of maraging steel 22 may be cooled to a temperature below the austenite solutionizing temperature, such as ambient (approximately 72° F. or 22° C.). However, other cooling temperatures may be selected.
After the thermomechanical processing step 24, the workpiece of maraging steel 22 is subjected to a direct aging step 26. For instance, the direct aging step 26 includes heating treating the workpiece of maraging steel 22 at an aging temperature to form precipitates within the martensitic microstructure. The term “direct aging” refers to aging the workpiece of maraging steel 22 without conducting any intervening heat treatments between the thermomechanical processing step 24 and the direct aging step 26. In some examples, any processing that occurs between the thermomechanical processing step 24 and the direct aging step 26 is conducted at temperatures no greater than room temperature (approximately 72° F. or 22° C.). In others examples, any processing that occurs between the thermomechanical processing step 24 and the direct aging step 26 is conducted at temperatures no greater than the aging temperature of the direct aging step 26. Therefore, relative to any heat treatments, the workpiece of maraging steel 22 proceeds directly from the thermomechanical processing step 24 to the direct aging step 26.
The direct aging step 26 may be conducted at any suitable aging temperature and for any suitable amount of time, depending upon the desired end mechanical properties of the workpiece of maraging steel 22 and the material composition. For instance, the aging temperature may be 850° F.-950° F. (454° C.-510° C.). In one example, the direct aging step 26 includes heating the workpiece of maraging steel 22 at a temperature of about 900° F. (482° C.) for about nine hours to precipitation strengthen the martensitic microstructure. As can be appreciated by those skilled in the art, the aging time may vary depending on the desired degree of precipitation.
The thermomechanical processing step 24 and the direct aging step 26 of the method 20 need not be conducted serially. That is, the thermomechanical processing step 24 may be conducted at one facility or site, and the direct aging step 26 may be conducted at another facility or site. Thus, from the perspective of a single processor, the thermomechanical processing step 24 of the method 20 may be regarded as receiving the workpieces of maraging steel 22 that have already been thermomechanically processed, and subsequently conducting the direct aging step 26.
As illustrated in Table I below, processing the workpiece of maraging steel 22 according to the examples disclosed herein provides desirable mechanical properties compared with control specimens that are solution heat treated between thermomechanical processing and aging. That is, the control specimens include an additional step of heat treating compared to the example method 20. As can be appreciated from Table I, the workpiece of maraging steel 22 (Directly Aged Specimen) processed according to method 20 provides a higher 0.2% yield strength and ultimate tensile strength than the control specimen. The % elongation of the workpiece of maraging steel 22 is somewhat lower than the control specimen; however, for some end uses the loss of elongation may not be a significant factor. The hardness of the directly aged workpiece of maraging steel 22 and the control specimen are each between about 56-58 Rc.
After the thermomechanical processing step 24, the workpiece of maraging steel 22 is subjected to the direct aging step 26 in a chamber 40 at an appropriate aging temperature for a predetermined amount of time. As can be appreciated, multiple workpieces of maraging steel 22 may be aged in the chamber 40 at the same time.
After the direct aging step 26, the workpiece of maraging steel 22 is removed from the chamber 40 and allowed to cool in air for example to a temperature less than 850° F. (454° C.), such as room temperature. Once cooled, the workpiece of maraging steel 22 may be considered to be a component of maraging steel 42. The component of maraging steel 42 may be subjected additional processes to produce an end component, such as finishing, coating, etc.
In general, the size of the grains 50 and the presence of the precipitates 52 contribute to the improved mechanical properties as illustrated in Table I. For comparison, martensitic grains of the control specimen of Table I are generally larger in size and may contribute to the lower ultimate tensile strength and 0.2% yield strength. Thus, the disclosed method 20 provides a desirable combination of forming and heat treating the workpiece of maraging steel 22 to obtain improved properties, and an economic process that eliminates the need for a heat treatment step between the thermomechanical processing step 24 and the direct aging step 26.
Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.
The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.
