NSTT - A Promising Tool for Properties Enhancement of Heat-Treated Steels
As it was originally designed, prolonged proprietary research with application of NSTT on ferrous and nonferrous rolled superalloys showed improved oxidation and corrosion resistance after multiple high-temperature, cyclic exposures to 2000°F (1093°C). During discussion of these results, the CEO of a Tier I OEM supplier presented the case of a very expensive government recall due to premature failure of its suspension coil springs. The OEM was searching for a solution by putting an inert plastic coating on the finished spring to avoid the perceived culprit of atmospheric corrosion. In an exploratory series of tests, NSTT was partially (2/3) applied to the coil-spring, alloy-steel bar samples taken from the OEM plant, and the samples were subjected to simulated heating conditions. The results are summarized in Figures 1 and 2.
While the untreated areas showed surface oxidation, scaling and bubbling at the end of the 1680°F (915°C) heating cycle, the NSTT-treated areas appeared to be free of surface degradation. The microstructures further revealed widespread penetrating oxidation, decarburization and grain-growth in the untreated areas (A), while the areas treated with NSTT (B) appear to have stopped the degradation and maintained its original state. Figure 3 shows the enlarged view of the bubble at the untreated area after heating for 15 minutes at 1680°F.
Looking through the “peephole” of the oxidized bubble on the untreated area, the smooth surface of the leftover body of the steel is observed. All over the world, at rolling mills or heat-treatment facilities, the blisters fall off on the floor, making a mess. This is accepted as a “nuisance” of the practice. In most steel mills, this substantial debris is even collected on a regular basis and used as an oxidizing agent to burn the undesirable elements in the steelmaking process. This fact reveals the universal acceptance of the oxidation, scaling and bubbling of steel upon heating at high temperatures. With NSTT treatment, this surface degradation might be a thing of the past.
Though in its embryonic stage of development, NSTT is fundamentally projected to be effective in many industrial applications.
Potential Industrial Applications of NSTTThe significant difference in superficial appearance and the subsurface microstructures of the alloy – with and without NSTT – were studied to evaluate projected NSTT efficacy for potential industrial applications such as:
- Alloy-steel parts – suspension coil and spring, catalytic-converter parts including the jacket and the exhaust system – used by the automobile and aerospace industries, which are heat treated for the attainment of peak-properties
- Heat-treatment baskets, which lose a microscopic layer of oxide scale every time they are used
- Turbine blades, shrouds and exhaust systems for aerospace
- All cutting tools
- Hot-rolled steel to reduce or eliminate scale and bubble formation on slabs or any hot-rolled product during heating. The carbon footprint of a steel plant should be considerably reduced by NSTT since it essentially stops decarburization during heating.
Fundamental Metallurgical Basics of NSTTCarbon is known and accepted as the lifeblood of steels. It forms hard carbides with iron, which dissolve into the austenite on heating. Upon cooling, various transformations occur based on the cooling rate. This fundamental process, called heat treatment, produces the desired strength, hardness, fatigue and all of the important characteristics of the alloy. But if carbon burns out (decarburization), everything changes, and all desired properties go downhill.
Most industrial high-temperature, continuous processes of mass production are done essentially in air, making decarburization inevitable. NSTT protects the alloy from surface and internal oxidation, decarburization and grain growth during industrial mass-production heating – reheating, heat treatment or rolling.
The critical elements, which are chosen according to the alloy to be protected, comprise the “medium.” One of the proprietary aspects of the NSTT process application, these elements are “nanolayered” during the process in the proprietary patent-pending “NSTT chamber.”
Metallurgical Principles of Heat TreatmentFigure 4 shows schematically the most important slope in the iron-carbon equilibrium diagram, which determines the temperature required to reach the austenitizing zone for heat treatment.
The ferrite stabilizers – Cr, Mo, V, Ti, Si and Zr – shrink the zone, raising the necessary hardening temperature. The austenite stabilizers – Ni, Mn, N, Cu, Co, and C – lower the slope to lower the temperatures.
Automotive Coil Spring
The OEM hot-coil steel contained about 0.60% C. Figure 2 shows the extent of decarburization – about 80% or more on the surface and gradually less below the surface up to a depth of about 0.01 inches. Essentially, the bar is now composed of concentric rings of various carbon content, with the lowest at the surface and reaching about 0.60% at a depth of about 0.01 inches from the surface.
Figure 4 shows the effects of this decarburization on the hardening temperature and the final transformation products upon cooling and tempering. There will be various amounts of soft ferrite content in the so-called “hardened” coil spring – maximum being at the surface – ruining its expected properties. This was the reason the coil spring did not meet its intended properties. NSTT avoids this consequence by eliminating oxidation and decarburization.
The Effects of NSTT on PropertiesOxidation, decarburization, scaling, bubbling, grain growth – all undesirable consequences of heating in open or partially closed air – have been known agents of degradation of all important steel physical and mechanical properties. NSTT appears to protect steel, and it even improves the properties originally intended for the alloy. The elimination of decarburization alone contributes significantly to the heat-treated properties. All changes described in this limited number of experiments indicate only qualitative results. Ongoing research will equate these effects to quantitative, measured improvement. The fact that two major OEMs here and abroad are interested in probing NSTT for their industrial applications (see sidebar), however, indicates its promising potential value.
NSTT May Eliminate Shot/Stress Peening
“Shot peening improves the fatigue properties of alloy-steel products” – common knowledge among metallurgists – is universally accepted and practiced globally for products like suspension coil and leaf springs. It creates a compressive stress over the surface to partially counteract the damaging effects of decarburization. But it cannot bring back what the lost carbon would have contributed toward the products’ ultimate intended mechanical properties.
Since NSTT stops decarburization, it may be reasonably concluded that the universally practiced, labor- and time-consuming, costly process of shot/stress peening should be permanently removed from the processing plants that adopt NSTT for all heat-treated products prior to heating or heat treatment.
Atmosphere Control vs. NSTT
Question arises as to the feasibility of controlling the atmosphere during heating to stop the degradation caused by high temperature. Experience indicates that this solution is only partially effective. Even with positive pressure and using a very costly mixture of inert and deoxidizing gases, it is virtually impossible to create a sealed atmosphere in a chamber that requires repetitive entry and exit mechanism for the parts. Moisture creates an additional problem, especially in high-humidity countries like India. The easiest alternative, NSTT, overcomes all of these negatives.
ConclusionAdditional research is being conducted, here and abroad, to establish NSTT as a new tool to achieve measurable improvement in heat-treated properties of steel. Yet it is possible to project that, when accepted and implemented, NSTT may significantly impact the automotive, steel and aerospace industries by cutting cost, increasing production, reducing rejects and recalls, enhancing high-temperature resistance and allowing lower-cost alloys to perform the functions of higher-cost alloys for billions of parts and accessories. IH
For more information: Dr. Subrata Ghosh, chairman and chief engineer, NanoTech Metallurgy, Inc., 20317 Willowick Dr., Southfield, MI 48076; tel: 248-353-2304; e-mail: nano firstname.lastname@example.org
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: oxidation, decarburization, grain growth, austenite, nanolayer, shot peening, atmosphere control
SIDEBAR: Automotive OEMs Put NSTT to the TestAmong many of its applications, one major U.S. OEM, following its success in suspension coil experiments, is currently testing NSTT for its exhaust and catalytic-converter components. Another major OEM from India expressed active interest in NSTT for their coil and leaf suspension springs for which laboratory experiments are continuing.
NSTT may impact the steel industry in each and every process where reheating is employed prior to rolling, especially in the strip mill where a little patch of stubborn scale may roll into “a mile-long defect,” causing final rejection of the entire coil. This costly problem is prevalent among flat-roll producers of automotive-body panels worldwide. This is indicated by the fact that the author spent nearly one year of consultation with a major German producer to sort out the variables and find its multifaceted solution. A simple NSTT treatment prior to reheating the slabs may stop the scaling altogether. The consequent national and global benefits are obvious.