Hi KevinC,

As a general rule, Heat input control and the tightly control of inter-pass temperatures do play a very important part in maintaining most of the mechanical properties in thick, multi-pass welded joints... However certain mechanical properties that are crucial to maintain for a specific application will more than likely change, and usually not for the better if nothing is done to either minimize the loss of, let's use ductility or to prevent the increase in hardness for example, or to control the cooling of the metal once the welding has stopped for each pass... Now toughness does increase in multi-pass welds

In other words, control the temperatures using various techniques that may work on certain application but not so well in others has to be taken into consideration... Post Weld Heat Treatment or, PWHT is also used to regain certain mechanical properties... And yet, one must also consider what type of metal alloy is being treated because again, what works well with let's say carbon steels may not work as well with Aluminium for example... And there are certain metal alloys that are multiphase and have the ability to change from one phase to another, or to lose a phase which could be detrimental also... In terms of mechanical properties in carbon and low alloy steels especially toughness, multi-pass weld metal is usually better than in single pass weld metal for these reasons:

1.) The reheat thermal cycle of each subsequent welding pass normalizes and refines portions of the microstructure in previous weld metal...
2.) Subsequent weld runs temper the previous weld metal and reduce the residual stresses from previous runs...
3.) The total energy input per pass is reduced, Which is helpful in limiting the amount of grain growth...
4.) Previous weld passes provide a certain pre-heat that slows down the cooling rate of the subsequent pass in carbon and low alloy steels...

So without getting too deep into an explanation... The bottom line is that heat input and inter-pass temperatures must be tightly controlled in order to maintain the desirable mechanical properties necessary for a multi-pass welded joint... And the welding process & specific procedure chosen is also a consideration that must be seriously adhered to if the application is to have a successful result...  There is a caveat with certain high strength steel alloys involving the control of hydrogen diffusion that must be seriously taken into consideration also...

However, if there are no controls involved then the improvements are quickly nullified and reduced if heat input and inter-pass temperatures as well as cycling and controlling the cooling rate is ignored and left to create a condition where the weld metal has lost ductility and toughness as well as other properties, and has increased or decreased in certain high strength alloys in hardness and  brittleness...

You asked for some reading that covers this topic correct? Well, I have a few articles and papers that I can rustle up for your viewing pleasure... Now, if you want some reading material for a specific application then you need to post it first because unfortunately and at times fortunately I cannot read peoples minds so, please let me know beforehand and I'll see what I can do for you...

Here's an oldie but goodie article published in the AWS Welding Journal way back in 1987 titled: "Effect of the Number of Passes on the Structure and  Properties of Submerged Arc Welds of AISI Type 316L Stainless Steel. As the number of passes increased, the hardness and tensile strength increased, while the ductility and toughness decreased."
Here's the link:    http://www.aws.org/wj/supplement/WJ_1987_05_s147.pdf

Here's a paper titled: Influence of Welding on Steel Weldment Properties by C.C. Chen and Abe Pollack of Microalloying International Inc. he .pdf comes from the MIT server of their Material Science Department as part of their welding engineering course that was once taught by T.W, Eagar who is one of my favorite authors and researchers in the science of welding and who has published many relevant papers as well as many research studies who is very well respected and well known as one of the best that ever was...
Here's the link to the article:    http://eagar.mit.edu/handouts/H-3371-17.pdf

Here's a Doctoral Thesis Report written by Kenneth Hakansson in August 2002 for the Division of Welding, Department of Production Engineering, Royal Institute of Technology (KTH) titled: Weld Metal Properties for Extra High Strength Steels... Here's the link below:

http://kth.diva-portal.org/smash/get/diva2:9199/FULLTEXT01.pdf

Here's another oldie but goodie from way back in 1972 in the monthly aWS Welding Journal for I believe the month of October... The paper was written by S. S. Strunk and R.D. Stout... Titled: "Heat Treatment Effects in Multipass Weldments of a High-Strength Steel" Here's the link below:

http://www.aws.org/wj/supplement/WJ_1972_10_s508.pdf

Here's one from a friend in the UK who has written papers from here to the moon at the very least teaching and conducting much metallurgical research involving welding @ The University of Cambridge Department of Materials Science and Metallurgy, titled: "Complete Reaustenitisation in Multi-Run Steel Weld Deposits" Written by: R.C. Reed, H.K.D.H. Bhadeshia... Here's the link:    http://www.msm.cam.ac.uk/phase-trans/2002/complete.reaustenitisation.pdf

Here's "A Model for Multipass Steel Welds" also written by R. Reed and H. K. D. H. Bhadeshia... Here's the Abstract: "A model has been developed which is capable of describing the thermal cycles occurring in multipass welds. The method involves a calculation of the size and shape of the single bead-on-plate weld. The isotherms beneath the weld centre-line are estimated using an analytical solution for the problem of heat-flow into a thick plate. The volume of metal deposited per unit length of weld, i.e. the reinforcement, is then assumed to adopt the shape of a spherical cap on the surface of the plate. For multipass welds, the model applies the heat-flow equations as each bead is deposited. The model has been calibrated for a 2 1/4 Crl Mo multipass weld of a type commonly used for joining steam-pipes in a modern steam-generating power plant. The method has been used to investigate theoretically the effect of interpass temperature, welding current and Ac3 temperature on the fraction of weld microstructure which becomes reaustenitised during fabrication of the weldment. Where possible, the results are rationalised by comparison with experimental observations." This was written for: Acta Metallurgica et Materialia, Vol. 42, 1994, 3663-3678. Here's the link to the front page... Once there, download the zip archive in the page and open it... Here's the link:    http://www.msm.cam.ac.uk/phase-trans/abstracts/weld.reed.html

Here's another research paper also from the University of Cambridge in the UK and again written by Harry Bhadeshia and for this paper, co-written by J.R.Yang titled: "Reaustenisation in Steel Weld Deposits" The Abstract starts with: "The process of reaustenitisation in weld deposits, beginning with a microstructure of acicular ferrite and austenite, has been studied in order to enable the prediction of the reheated microstructure of welds. The transformation mechanism by which the original acicular ferrite formed is found to strongly influence the reaustenitisation process. The reverse transformation from ferrite does not occur immediately when the temperature is raised, even though the alloy may be in the ferrite plus austenite phase field. Reaustenitisation only begins when the carbon concentration of the residual austenite exceeds its equilibrium carbon concentration. This is a direct consequence of the fact that the acicular ferrite transformation ceases before the lever rule is satisfied. A theory has been developed, which explains the experimental data, including the fact that the degree of reaustenitisation varies with temperature above the Ae3 curve." Here's the link below & Download the .pdf:

http://www.msm.cam.ac.uk/phase-trans/2002/koo.reaust.html

Here's a report form Nippon in Japan Steel where they investigated The validity of α as an index of heterogeneity of microstructure was verified, and the clarification of the influence of heat input and interpass temperature on the toughness of weld metal...C Metallurgical controlling factors for toughness of multi-layered weld metal in
beam-to-column connections were investigated by considering the heterogeneity of microstructure as a dominant factor of scatter of Charpy absorbed energy. The fraction of as-welded zone of weld metal included in notch part of the test piece for Charpy impact test was defined as fraction of columnar structure (α  ).
                                                                                                                                                                                             C
Here's the link:http://www.nssmc.com/en/tech/report/nsc/pdf/n9513.pdf

Here is a more recent paper that is eerily similar in content and is written by:Masao Toyoda, Masahito Mochizuki of Department of Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan...Received on September 8th, 2003; revised October 17th, 2003; accepted October 20th, 2003. Titled: "Control of mechanical properties in structural steel welds by numerical simulation of coupling among temperature, microstructure, and macro-mechanics..." In this paper, numerical simulation method of coupling analysis of temperature, microstructure, and stress – strain fields has been developed and applied to investigate the effects of heat input and inter pass temperature at multi-pass welded joint of beam-to-column connections on the strength and fracture... Here's the link below:

http://iopscience.iop.org/1468-6996/5/1-2/A37/pdf/STAM_5_1-2_A37.pdf

I have too many more to share and to little time to post them so I'm going to stop here because I have Pulmonary therapy later today... I'll post some more later today after I rested from the therapy...  Enjoy the reading!

Respectfully,
Henry

Hi Kevin!

Yes what you are stating is basically correct in a broad sense! There is a member on here who has a vast knowledge of links for further info! ssbn727 use the search facility!

Okay, I didn't see it listed in all of Henry's references so here is another resource to consider:  The Welding Handbook, Vol 1: Welding Science and Technology.  You can get it through the AWS Bookstore.  A very good read on many of these issues. 

He Is In Control, Have a Great Day,  Brent