Unmixed zones

All methods of welding stainless steel with a filler metal prodnce a weld fnsion bonndary consisting of base metal that has been melted, but not mechanically mixed with filler metal and a partially melted zone in the base metal. The weld fusion boundary lies between a weld composite consisting of filler metal diluted by base metal and the HAZ in the base metal. The width of the unmixed zone depends on the local thermal conditions along the weld fusion line. For a GTA welding process, the zone is most narrow at the weld face and is broadest near the middle of the weld thickness. 

An unmixed zone has the composition of base metal, but the micro-structure of an autogenous weld. The micro-segregation and precipitation phenomena characteristic of autogenous weldments decrease the corrosion resistance of an unmixed zone relative to the parent metal. 

Unmixed zones bordering welds made from overalloyed filler metals can be preferentially attacked when exposed on the weldment surface. 

The potential for preferential attack on unmixed zones can be reduced by minimizing the heat input to the weld and/or by flowing molten filler metal over the surface of the unmixed zone to form a barrier to the service environment. 

Care must be taken in this latter operation to avoid cold laps and lack-of-fusion defects. In both cases, preferential attack is avoided as long as the surface of the unmixed zone lies beneath the exposed surface of the weldment. 

---

The dynamic changes actually lead to the formation of a pulse, which is more or less elongated depending on the frequency [91]. This pulse is followed by a hardly deformed zone so that mixed and unmixed zones are created in an alternating... 

Even when suitable fillers are used, preferential pitting attack can still occur in an unmixed zone of weld metal. High heat input welding can leave bands of melted base metal close to the fusion line. The effect of these bands on corrosion resistance can be minimized by welding techniques that bury unmixed zones beneath the surface of the weldment. 

We have also measured the lithium abundances in the samples of unmixed and mixed stars [6]. When low mass stars, such as those in our sample, evolve through the red giant branch, the degree of dilution of the lithium increases as the convective zone penetrates deeper and thus we expect a decline of the lithium abundance. In the mixed stars the lithium has never been detected, the upper limit of the lithium abundance is log N(Li) < 0.0, on the contrary in all the unmixed stars but one, the lithium line is visible and log N(Li) is > 0.20. In these stars as expected, the lithium abundance decreases when the gravity decreases (Fig. 3-b). 

Figure 5.17. A laminar diffusion flamelet occurs between two regions of unmixed fluid. On one side, the mixture fraction is unity, and on the other side it is null. If the reaction rate is localized near the stoichiometric value of the mixture fraction st, then the reaction will be confined to a thin reaction zone that is small compared with the Kolmogorov length scale.

Figure 5.20. The flamelet model requires the existence of unmixed regions in the flow. This will occur only when the mixture-fraction PDF is non-zero at = 0 and = 1. Normally, this condition is only satisfied near inlet zones where micromixing is poor. Beyond these zones, the flamelets begin to interact through the boundary conditions, and the assumptions on which the flamelet model is based no longer apply.

Macroperthites. When the unmixed K-rich and Na-rich zones may be observed and characterized directly under the microscope ... 

In diffusion combustion of unmixed gases the combustion intensity is limited by the supply of fuel and oxidizer to the reaction zone. The basic task of a theory of diffusion combustion is the determination of the location of the reaction zone and of the flow of fuel and oxidizer into it for a given gas flow field. Following V. A. Schvab, Ya.B. considered (22) the diffusion equation for an appropriately selected linear combination of fuel and oxidizer concentrations such that the chemical reaction rate is excluded from the equation, so that it may be solved throughout the desired region. The location of the reaction zone and the combustion intensity are determined using simple algebraic relations. This convenient method, which is universally used for calculations of diffusion flames, has been named the Schvab-Zeldovich method. 

M 83] [P 72] The inlet flow rate in one feed channel was set constant at 1.0 mm s-1 and the flow rate of the other inlet was varied periodically in time with 1.0 + 7.5 sin (5 2 n t) mm s 1 [26], The flow pattern is again of tri-lamellae type with two outer unmixed and an inner diffuse, mixed layers however, the diffuse mixed zone is now much larger than in the constant flow case (see Figure 1.171 and Constant flow rate for both inlets). 

Welded microstructures can be extremely complex and often change drastically over a very short distance. The fusion zone or weld metal is a dendritic structure that has solidified from a molten state. Bordering the fusion zone are transition, unmixed and partially melted zones, and the heat-affected zone (HAZ). These zones can be reheated and altered by subsequent weld passes, in multipass welding. For alloys with structures that depend strongly on thermal history, such as steels, the final microstructure can be extremely complex. Since welded structures are often quite susceptible to corrosion, overalloyed filler metals are often used to enhance the weld corrosion resistance. For stainless steels with sufficiently high carbon content, sensitization in the HAZ is another major problem. (Frankel)5... 

Diffusion flaming combustion of polymeric materials may be compared with that of unmixed gases. Zeldovidi related the extinction limits of the diffusion flame with the reactant concentration (mass flow rate) in the chemical reaction zone and its cooling. At the extinction limit the intensity of diffusion combustion of unmixed gases is at its maximum corresponding to the combustion intensity of a premixed stoichiometric mixture of the same gases. 

Those which are described by Dioscorides (1. 5, c. 90), and which are mined in Egypt. They have a black ground, are full of colour, and are hard, crumbling, smooth in quality, unmixed with any dross, and not encircled by zones. This species is found among the Hematites obtained from our mines, but very rarely I have barely met with a couple, and to the vulgar they are wholly unknown. 

In a restricted sense, a diffusion flame may be defined as a nonpremixed, quasisteady, nearly isobaric flame in which most of the reaction occurs in a narrow zone that can be approximated as a surface. This is the class of flame problems that will be discussed in the present chapter. The coupling-function formulation (Section 1.3) provides a convenient framework within which these problems can be studied. Coupling functions usually are more useful for initially unmixed systems than for initially premixed systems because, as we shall see, the chemical reaction rate is often of negligible importance in answering certain questions raised for diffusion flames. 

---