Interestingly, dimensional growth is nearly the same with the grain direction as across the grain direction. This is attributed to the absence of primary carbides. The DC53 steel with 8 percent chromium will grow dimensionally more with the grain direction than across it. D2 will have even more dimensional variance.

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0.6-1.43% carbon (C)/0.28-0.32% silicon (Si)/0.4-0.43% manganese (Mn)/5.09-11.92% chromium (Cr)/ 0.97-1.01% molybdenum (Mo)/0.27-0.31% vanadium (V).

The third-generation low-carbide grade shows no primary carbides, only fine secondary carbides formed in secondary hardening at high-temperature tempering (see Figure 4). The small dark spots are particles formed by manganese and a small amount of sulfur, which results in a dramatic improvement in machinability compared to D2.

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The inherent flaw in all these “D” chemistries is that the high carbon and carbide-forming elements, especially chrome, led to the formation of large, irregularly shaped, undissolved carbides in the matrix (see Figure 2).

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Trial dies of D2, a third-generation low-carbide steel, and an 8 percent chromium/sulfur steel were run to 1.5 million hits. Both the D2 and steel block dies with 8 percent chromium had pieces of the die missing. The third-generation low-carbide steel was intact even after the results were repeated with and without PVD coatings.

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Advancements in production methods helped improve steel quality and subsequent tool performance. The base chemistry was modified to attempt to extend tool life (see Figure 1).

Additionally, dimensional growth in heat treat is a function of the percentage of carbides in the steel. As the carbide runs in the rolled grain direction, more dimensional growth occurs with the grain direction than across it.

A third characteristic of the low-carbide grade is its dimensional stability. Tempered at 932 F, dimensional change is a modest 0 to -0.05 percent shrinkage. When tempered at higher temperatures, 970 to 1,000 F, dimensional growth is +0.05 to +0.10 percent.Interestingly, dimensional growth is nearly the same with the grain direction as across the grain direction. This is attributed to the absence of primary carbides. The DC53 steel with 8 percent chromium will grow dimensionally more with the grain direction than across it. D2 will have even more dimensional variance.Two other ways in which the low-carbide grade has improved over its predecessors are in impact strength and fatigue strength. Its impact strength approaches that of S7. Testing fatigue strength with a rotating bend test at like load shows DC53 with longer life by a factor of 10 times the life of D2, and the low-carbide grade has a lifespan of 10 times that of DC53.As to which steel is best for a given application, the DC53 with 8 percent chromium steel has somewhat better wear uncoated and better temper resistance than the low-carbide grade with 7 percent chromium. The low-carbide grade has higher impact strength and higher fatigue strength. Both will substantially outperform D2 in both coated and uncoated applications. Both offer substantial improvement in tool life compared to D2. Grade selection depends on the failure mode of the current die steel. Figure 1 Microstructure of D2 shows large carbides.Case ExampleWhen a major automaker was trying to select a die steel to stamp HSS for a significant model change, its final testing for blanking dies came down to the third-generation low-carbide grade and an 8 percent chromium steel with the following chemistry/percentages:C 1/Si 1.5/Mn 0.4/Si 0.05/Cu 0.35; Ni 0.45; Cr 8/Mo 0.9/V 0.1/Al 0.4.D2 was included as a control steel.The test was run blanking 1.6-mm-, 440-MPa HSS. Each leg of the square blanking die was made of a different steel. Two were D2, one was the low-carbide grade, and the fourth was the steel with 8 percent chromium. The die was put into service and run 1.5 million hits before inspection (see lead image).After 1.5 million hits, both the D2 and steel with 8 percent chromium had pieces missing, while the low-carbide grade was intact. The results were repeated, both with and without PVD coatings. The low-carbide grade was selected for all tooling. Slide Show | 5 Images

Forming HSS presents serious challenges. For forming sheet steels above 1,470 MPa, hot stamping is the current practical method. But wear, chipping, fracture, and galling occur during forming of the lower-strength sheet steels from 440 MPa to 1,470 MPa. Coatings such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and thermal diffusion (TD) must be applied to tool steels to prevent galling.

The researchers concluded that the following chemistry/percentages exhibited the best combination of hardness, fatigue strength, impact strength, and wear resistance plus the favorability for weld repair (see Figure 3):

These carbides help with wear resistance only. They reduce impact strength and fatigue strength by acting as a stress riser. Viewed under magnification, a cutting edge of a working D2 die shows small microchips. These occur as a result of the matrix’s fatigue failure around the primary carbide, which causes the carbide to pop out. This left small, jagged areas that can cause further chipping, galling, and cracking. The large irregular carbides also cause cutters and grinding wheels to wear rapidly.

Emily Wilkins, founder of Marketing Metal, helps job shops, machine shops, metal fabricators, custom equipment builders, and other...

In 1912, D2 cold-work die steel was introduced to the English tool building market. At that time it represented an advancement in cold-work tooling applications. The chemistry allowed for hardening to HRC 60 by either salt bath or air quench. D2 was sufficiently tough and showed good wear properties for the applications of the period.

The improved DC53 tool steels with 8% Cr/2% Mo have proven to hold their hardness well and are a good substrate to support the various coatings. However, the metallurgical team sought to develop a hard steel that would support coatings well, like the DC53, but with even higher impact strength. It formulated a chemistry to further reduce undissolved primary carbides. The chemistry they arrived at was 0.70% C/7% Cr/1.5% Mo/0.15% V.

As to which steel is best for a given application, the DC53 with 8 percent chromium steel has somewhat better wear uncoated and better temper resistance than the low-carbide grade with 7 percent chromium. The low-carbide grade has higher impact strength and higher fatigue strength. Both will substantially outperform D2 in both coated and uncoated applications. Both offer substantial improvement in tool life compared to D2. Grade selection depends on the failure mode of the current die steel.

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After 1.5 million hits, both the D2 and steel with 8 percent chromium had pieces missing, while the low-carbide grade was intact. The results were repeated, both with and without PVD coatings.

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The Fabricator is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The Fabricator has served the industry since 1970.

Two other ways in which the low-carbide grade has improved over its predecessors are in impact strength and fatigue strength. Its impact strength approaches that of S7. Testing fatigue strength with a rotating bend test at like load shows DC53 with longer life by a factor of 10 times the life of D2, and the low-carbide grade has a lifespan of 10 times that of DC53.

In the early 1980s, a team of research metallurgists in Japan led by Dr. Kunio Namiki began experimenting with chemistries to reduce the problems caused by excessive, uncontrolled primary carbides. Other mills had introduced various powder metals to try to overcome these problems. But the metallurgical team determined that the expense of powders and their inability to be welded properly rendered them less than ideal and sought an alternative. They experimented with the following chemistries and percentage ranges:

The test was run blanking 1.6-mm-, 440-MPa HSS. Each leg of the square blanking die was made of a different steel. Two were D2, one was the low-carbide grade, and the fourth was the steel with 8 percent chromium. The die was put into service and run 1.5 million hits before inspection (see lead image).

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The new grade, named DC53, was introduced to the Japanese market in 1987. Soon the new grade had taken many thousands of annual tons of market share from the D2 (SKD 11) and M2 (SKD51) markets. Grade DC53 remains a major cold-work die steel in the industry today.

The reduction of primary carbides also facilitated faster machining and grinding. In addition, the alloy performed better at higher draw temperatures—970 to 1,050 degrees F—than D2. This resulted in secondary hardening by precipitating out very fine carbides into the matrix. Additional benefits were less stress for better stability when undergoing electrical discharge machining (EDM); reduced retained austenite; and higher obtainable working hardness (up to HRC 64) than D2.

Corporate average fuel economy (CAFE) standards mandate that automakers lighten vehicles drastically to meet mileage and emissions targets. That, combined with the need for better crash energy management, has led to a dramatic increase in the use of high-strength steels (HSS) to form automotive structural components.

A second characteristic of the low-carbide grade is its higher degree of hardness—HRC 61-62—than can be obtained with D2—HRC 58-59—when it is tempered at 932 F (500 C). This supports PVD coatings better. If the coating temperature is higher than 932 F, raising the tempering temperature to between 968 F (520 C) and 1,004 F (540 C) still strengthens the steel to between HRC 59 -61 for excellent support of the coating.

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When a major automaker was trying to select a die steel to stamp HSS for a significant model change, its final testing for blanking dies came down to the third-generation low-carbide grade and an 8 percent chromium steel with the following chemistry/percentages: