Stress induced ageing

Material brazing gives rise to compression around the zone brazed. Even though the rapid structural transformation is well known, it is not known whether the stress induces a different ageing behaviour.

Literature research

Starting from the high temperature austenite phase, the ageing in steel goes through three distinct phenomena. 1, decay of martensite created initially through diffusionless transformation of austenite. 2, carbon segregation around/in dislocations. 3, precipitation.

The martensite is a phase in which carbon atoms occupy a single type of interstitial site. This is underpinned by the increase in the yield stress of Fe aged at 20 °C that was reported to be proportional to the C content up to 0.11 at%C Wilson (1959). Besides, in this paper, the increase of the yield stress was greater if the prestraining load was not removed, which may indicate that there is some sort of C ordering that is independent from the atmosphere formation.

The upper yield point (which they call "the size of yield point") is independent of the solute content if it exceeds a certain amount (of 0.001 at%C) Wilson (1960). The density of the steel was not written in the article, but as it was 4% elongated, I assume it would be around 1014 m-2 Dollar (1984). The atmosphere locking, on the other hand, starts at 0.0001 at%C (for how much dislocation density?). According to Cottrell (1949), for the dislocation density of 1012, the full yield point was observed when around 0.01 wt%C was absorbed by dislocations. For the dislocation density of 108, it was around 2.0x10-6. This assumption is especially underpinned by the fact that the specimens were aged only for 2 minutes in this experiment at 20 °C, whereas it was observed via thermoelectrical power Lavaire (2001), that the iron ageing for ULC, i.e. 0.004 wt%C converges well after a couple of months.

In 9.5 wt%Mn steel Milititsky (2005), it was shown that cyclic load-reload and load-unload-reload with 2 min ageing time did not have an effect on the flow stress, though this paper does not state the C content.

Ageing inside the martensite-ferrite dual phase steel shows no particular effect on the ageing behaviour Waterschoot (2003). This is particularly important since the martensite phase within in this steel is created from austenite-martensite transformation, which involves a volume increase of 2 to 4 % Moyer (1975). The C atoms are expected to form Cottrell atmospheres around dislocations and grain boundaries (then later precipitate) Calcagnotto (2011). Here the martensite-ferrite structure is probably still existent after ageing as half of it transforms after ~50,000 s at 80 °CWaterschoot (2006).

Hardning also arises from martensitic transformation, that is realized by quenching the material and keep it under subzero temperature Kraus (1999). The carbon diffusion can be suppressed in Fe-Ni-C but for Fe-C alloys and low-alloy steels, it cannot be done, because the Ms temperature is under zero in the case of Fe-Ni-C but not for Fe-C. It was even shown that the carbon mobility is sufficient to cause cementite precipitation in the martensite during quenching to room temperature (autotempering) Speich 1969.

Coarse second phased particles imbedded in martensitic matrices does not play an important role in hardning but it does for the fracture Kraus (1999).

Phosphorus seems to segregate under stress Song (2006), although whatever the reason is, it desegregate after around an hour at 500 °C. Yet even after desegregation the P boundary concentration is 400 times higher than in total P concentration. It is also noteworthy that the reduction of toughness due to P segregation ageing does not involve hardning, i.e. the material becomes brittle, without becoming harder.

Questions to answer

It was mentioned Wilson (1960) that the saturation effect "must be absent", due to some form of "precipitation", of the solute atoms collected by the dislocations. If so, it actually does not make so much sense to look for the atmosphere saturation. Cited papers not found. Is it true?

Ms temperature is the temperature at which martensite forms spontaneauously Fahr (1970). Why does it go down if the C concentration grows Kraus (1999)?

It was stated Kraus (1999), that the carbides do not play an important role in the hardning of martensite, but it does for the brittleness. Is it only for martensite or can it be stated for all phases?

Internal friction Blanter (2007)

It has been shown Garruchet (2007) that local stress modifies the activation energy, which means it affects also the diffusivity. Will this affect the behaviour of Cottrell atmosphere formation?

Questions answered

Prestraining of 4% elongation sufficiently "exceeds Lüders strain" Wilson (1960). Lüders strain is the strain at which a Lüders band appears? If so, is it the strain for the upper yield stress? → Lüders strain is the strain at which the Lüders band disappears Sylwestrowicz (1951). Hence, it is the highest strain value which gives the lower yield stress.

Yield propagation in a strain-aged steel is believed to be essentially similar to that in the annealed condition Wilson (1960). Probably this means nothing more than what it says...

Carbon Ordering

Among the three interstitial site types existent inside ferrite, the C atoms inside martensite occupy only intersititial sites of the same type. The nearest iron neighbours of an interstitial site being in the same axis, the presence of an interstial C atom leads to a tetragonal lattice distortion.

Unsorted stuff

But for any given angular configuration with respect to the dislocation line an interstitial atom in one type of site has a lower interaction energy (i.e. stronger binding to the dislocation) than in the other two sites. Evans (1973)

Steel Ageing

Steel is long known to be subjected to ageing phenomena that limit its life span. In the pioneering contribution of Cottrell and Bilby Cottrell (1949), it is shown that carbon atoms segregate around dislocations, which leads to the formation of the so-called "Cottrell atmospheres". These atmospheres hinder the dislocation motion, inducing, at macroscopic scale an increase in the upper yield stress and strain instabilities visible in the form of Lüders bands Hall (1970).

Formation of Cottrell atmospheres involves carbon diffusion, which takes significantly more time than Snoek ordering. This is observed in stress strain test at different temperatures Roberts (1970), where jerk flow (due to Snoek ordering) was observed at low temperature and serrated flow (Cottrell dragging) at higher temperature.

Other phases

Beetween around 700 and 1100 K, CO and HCOH are absorbed and dissociated on the Fe surface Restrepo (2016)

Literature research


In ultrafine-grained (UFG) ferrite steels, the strain hardening rate is allgedly very low Calgagnotto (2011). What is "strain hardening rate"? (It is very likely related to the fact that the coarse grained materials exhibit very high hardness and very low toughness. Maybe it's just another way to say the same thing).

Stress induced ageing
Carbon ordering
Steel Ageing