Carbon equivalent

In materials science, the carbon equivalent is a measure for assessing the weldability of steels .

definition

In materials science, the carbon equivalent is a measure for assessing the weldability of unalloyed and low-alloy steels. The carbon content and a variety of other alloying elements in steel influence its behavior. In order to assess weldability, the carbon content and the weighted proportion of the elements that influence the weldability of the steel in a manner similar to that which would be expected of carbon are combined into a numerical value in the carbon equivalent. A value of the carbon equivalent of less than 0.45% implies good weldability. Higher values require - depending on the processing thickness - the preheating of the material. From a value greater than 0.65, the workpiece is only suitable for welding with increased effort, since the formation of martensite can lead to cold or hardening cracks.

Calculation of the carbon equivalent

There is no generally applicable method for calculating the carbon equivalent and a different method is therefore used depending on the purpose.

Common are:

CEV (Carbon Equivalent Value)

Can be used from a carbon content of 0.18%, serves to estimate the necessity of preheating to avoid hardening cracks (standardized in EN 10025-1); the preheating temperature T ₀ can be approximated with the aid of special formulas. CEV = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5

CET (Carbon Equivalent Thyssen)

It is used to estimate the preheating temperature T _p (or interpass temperature T _i ) in order to avoid hydrogen-supported cracks - in particular, a higher initial temperature should result in a sufficiently long cooling period (so H _{2 can} sufficiently detach itself from the weld metal) CET = C + (Mn + Mo) / 10 + (Cr + Cu) / 20 + Ni / 40

The following relationship applies to the preheating temperature T _p :

{\ displaystyle T _ {\ text {p}} = 700 \ cdot CET + 160 \ cdot \ tanh \ left ({\ frac {d} {35}} \ right) +62 \ cdot HD ^ {0 {,} 35 } + (53 \ times CET-32) \ times Q-330}

Here, HD is the amount of diffusible hydrogen in ml / 100 g of the weld metal and the heat input with (in the case of electrical fusion welding processes). Here, the relative thermal efficiency and the welding speed are assumed. ${\ displaystyle Q}$ ${\ displaystyle Q = k \ cdot \ left ({\ tfrac {U \ cdot I} {v}} \ right) \ cdot 10 ^ {- 3}}$ ${\ displaystyle k}$ ${\ displaystyle v}$

PCM (Critical Metal Parameter)

Used for short cooling times and root welds: PCM = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 B

The alloy proportions are to be used in percent. As a rule, the carbon equivalent is specified in the material certificate so that the user does not have to make this calculation himself. However, under certain circumstances the usability of the CEV specified there for the specific application must be questioned.

Effects of high carbon equivalents

As the carbon equivalent increases, a multitude of problems can be expected when welding the workpiece, including:

Hardening and embrittlement of the workpiece in the heat affected zone

and caused by it

different types of cracks in the structure

Measures to increase the weldability

Preheating
Selection of a suitable seam geometry
Use of suitable electrodes

Welding cast iron

The carbon equivalent is also important for carrying out repair and construction welds on cast parts, as this value can be used to determine the preheating temperature during welding. For example, light-alloy cast steel parts have to be preheated to 250 ° C with a CE value of 0.6% in order to be able to weld them without defects. As with steel, the existing wall thickness of the areas to be welded is an important influencing factor.

criticism

The carbon equivalent is an auxiliary variable for the weldability. However, since this depends on a large number of factors, its exact definition depends on the material and the type of weld. The carbon equivalent is therefore not to be regarded as a universally valid value, but has to be tailored to the respective application. For example, many sources recommend the use of PCM for carbon contents below 0.18%.

Individual evidence

Web links

CEV calculator

[1] Roloff / Matek machine elements: weldability of steels, carbon equivalent and hardening cracks

[2] Comprehensive explanation of the applicability of carbon equivalents with calculation formulas .