Cryogenics in HSS


Introduction

Now first, what is cryogenics? Cryogenics is the process of exposing materials, in this case steels, to very low temperature, -320F, the temperature of liquid nitrogen.

In steels, cryogenics appears at first to be nothing more than an extension of the quenching process which forms martensite from austenite. Martensite forms during the cooling of austenite when the steel reaches the Ms point at which martensite starts to form and it continues until the Mf point which is where the martensite formation is completed. Now in steels like O1, as shown in the isothermal diagram on the right, the Mf point is above room temperature and thus cryogenics historically was not heavily advocated for such steels.

However as the alloying elements in steels increase, in particular the alloying elements which stabilize austenite, then the Mf point can be below room temperature and thus if the quench is stopped there then there can be very high levels of retained austenite. In high alloy steels which are austenized at higher temperatures, the Mf point can be below -150 F . Hence high alloy steels such as A2 and D2 were/are commonly advocated as strongly responding to cryogenics to ensure full martensite transformation and reduction of austenite.

In addition to the increased formation of martensite, it was discovered that cryogenics also caused the formation of a number of sub-structures, in particular the precipitation of very fine eta-carbides. It is no surprise then that many studies on HSS show marked improvement in wear resistance :

The presence of austenite in CHT specimen is the prime reason for the poor wear resistance exhibited by the specimen. Presence of austenite is inevitable in CHT specimens especially when the alloying elements are in higher proportion. Since AISI T42 HSS is a high alloy steel, the presence of austenite at the end of conventional heat treatment is mathematical certainty. Austenite being the softest phase present in the CHT specimen, wear took place only at these sights by ploughing action of hard asperities available in counter face. In the DCT specimen there was no trace of austenite. Apart from the above fine carbides of size less than 0.1 µ precipitated during the low temperature tempering at the end of cryogenic treatment, strengthened the martensite matrix by dispersion hardening. The above transformations that occurred during cryogenic treatment gave rise to increase in hardness value from 67 HRC at the end of CHT to 69HRC.Improvement in wear resistance of DCT specimen is mainly due to the above transformations that occurred in microstructure and hardness.

However when cryogenics is carefully and contrasted to conventional heat treating over a wide range of material tests including multiple wear resistance measurements, bend fracture tests, and lower impact toughness then the advocation of cryogenics is not so simple.

This experimental study investigates the properties of HS6-5-2 high speed tool steel subjected to deep cryogenic treatment (DCT) carried out at -180 °C. Microstructures of conventionally and DCT treated samples were examined with aid of scanning electron microscopy. The characteristic feature of deep cryogenically treated steel, distinguishing it from steel heat treated in a conventional way, was significant refinement of martensite plates. Material processed in this manner was found to exhibit slight decrease in hardness and considerable increase in impact strength. Deep cryogenic treatment caused the decrease of steel’s intensity of wear for about 36 %. However, this result was obtained during wear test carried out under a load of 100 MPa, while under four times higher load, slight worsening of tribological properties was observed. In the study, performance of deep cryogenically treated twist drills in drilling of ISO C45 constructional steel was evaluated in terms of tools in-service life. The assessment of the durability of drills was based on the occurrence of acoustic emission, as a symptom of obtaining the critical dulling. A general worsening of drills durability was observed after DCT treatment.

In this case the wear resistance gain was lost when a higher load was used and when actual tool life was examined it decreased. In addition use of cryogenics caused a loss in hardness and increase in bend fracture strength. The loss of hardness is a critical point and at first seems very surprising given the above commentary on the martensite transformation. Now why does this happen :

The decrease in hardness may be explained with different crystal morphology of martensite grain, and connected with this process change of strengthening mechanism and the formation of additional, micro-dispersive carbide precipitations (during tempering), which nucleation began during DCT.

In short, during conventional heat treatment of HSS, during the tempering the retained austenite transofrms to martensite which is a volume expansion and this reaction is argued to be partially responsible to prevent the softening in tempering. The logical response to this then is to adjust the tempering temperature when cryogenics is used. This has also been extensively studied.

The inflence of Deep Cryogenic Treatment (DCT) on the properties of four wrought and PM high speed steels was investigated. Hardness and (apparent) fracture toughness Ka were measured to highlight the possible inflence of DCT carried out before and after tempering. Dry sliding wear tests were carried using a block on disc confiuration. The properties of the two wrought steels, HS6-5-2 (AISI M2) and HS6-5-2-5 (AISI M35), highlight a well defied inflence of DCT. HS6-5-2 shows a remarkable improvement in abrasive wear resistance without any hardness increase. The most promising result is obtained by carrying out DCT before tempering. The opposite occurs by HS6-5-2-5, containing about 4.8%Co, whose wear resistance decreases in any treatment condition including DCT. PM steels show a less signifiant change in properties, since these are mainly controlled by the high amount of evenly distributed primary carbides which are not inflenced by DCT. A slight increase in wear resistance was observed just for HS6-5-3. A general worsening was observed for HS6-5-3-8, namely HS6-5-3 plus 8%Co. In the light of results here presented cobalt seems to play a negative effect with respect to the low temperature conditioning of martensite.

By adjusting the tempering temperature there is a significant increase in wear resistance and no loss in hardness. Note however the contrast when comparing the cobalt HSS.

Cobalt has many benefits in HSS, it keeps ferrite out of the as-quenched steel, it destabilizes austenite, it allows very high soak temperatures as it raises the solidus temperature and it increases hot hardness/strength. However it is very expensive, it can decrease toughness and it shows a stark contrast in reaction to cryogenics as the cobalt HSS react negatively to cryogenics. This could explain where there are some conflicting reports of properties of cryogenics on HSS.

Another possible reason could be revealed to be due to the influence on the length of cryogenic soak time.

Based on the results obtained in the present investigation, the following conclusions can be drawn.

  1. The bulk hardness of M2 HSS tool steels showed improvement in hardness as a result of cryogenic treatment.
  2. Mild-to-stable transition is observed at 4?hrs of cryosoaking time with consequent reduction in wear rate by 87%.
  3. Wear mechanism is clearly delineated into mild wear regime and stable wear regime with dominance of delamination wear mode and adhesive wear modes, respectively.
  4. In addition to primary and secondary carbides precipitation, the cryogenic treatment facilitates tertiary carbide formation which improves wear resistance remarkably.

The interesting thing in this study is that the hardness and corresponding properties can increase with cryogenics, however as noted in the image at the right they can also decrease. This is argued to be due to oswald ripening of the carbides. There is thus an oscillating process as carbides precipitate (which strengthen the steel) and then they coarsen (which weakens it). Thus there is a series of peaks and valleys.

Recap

In short, cryogenics can increase the material properties of HSS and can be a valuable addition to a heat treating cycle. However it can not be simply added to an existing cycle as it could readily degrade the steel. When cryogenics is added to a heat treating cycle then the austenization temperature, tempering temperature and time of soak all have to be consider and optomized to ensure the desired performance is achieved.


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Written: 17/01/2015 Updated: Copyright (c) 2015 : Cliff Stamp